FIFTH AMENDMENT TO OFFICE BUILDING LEASE AGREEMENT

Exhibit 10.21

FIFTH AMENDMENT TO OFFICE BUILDING LEASE AGREEMENT

This Fifth Amendment to Office Building Lease Agreement (this “ Amendment ”) is made and entered into this 18 th  day of April, 2007, by and between WESTCORE PYRAMID, L.P., a Delaware limited partnership (the “ Landlord ”), and EVOLVING SYSTEMS, INC., a Delaware corporation (the “ Tenant ”).

RECITALS

A.                                    Meridian Associates West, a Colorado general partnership, the predecessor-in-interest to Pacifica Holding Company, a Colorado corporation, the predecessor-in-interest to Mack-Cali Realty, L.P., a Delaware limited partnership, the predecessor-in-interest to Landlord, and Tenant entered into that certain Office Building Lease Agreement dated March 30, 1995, as amended by that certain Amendment Number One to Lease Agreement dated October 11, 1996 (“ First Amendment ”), that certain Amendment Number Two to Lease Agreement dated October 22, 1999 (“ Second Amendment ”), that certain Third Amendment to Office Building Lease Agreement dated July 17, 2002 (“ Third Amendment ”), and that certain Fourth Amendment to Office Building Lease Agreement dated May 24, 2004 (“ Fourth Amendment ”) (the Office Building Lease Agreement, the First Amendment, Second Amendment, Third Amendment and Fourth Amendment are hereinafter sometimes collectively referred to as the “ Original Lease ”), with respect to certain premises located within the office building (the “ Building ” ) located at 9777 Pyramid Court, Englewood, Colorado, as more particularly described therein.

B.                                      The term of the Original Lease is currently set to expire on May 31, 2007.  Tenant has requested, and Landlord has accepted, an extension of the term of the Original Lease.

C.                                      Landlord and Tenant desire (i) to extend the term of the Original Lease, (ii) to reduce the size of the existing premises, (iii) to establish the Base Rent and Base Operating Expenses for the extension term, and (iv) to provide other amendments to the Original Lease, all subject and pursuant to the terms and conditions set forth below.

AGREEMENT

NOW THEREFORE, in consideration of the premises and the mutual covenants hereinafter set forth, the parties agree as follows:

1.                                        New Premises Lease Term .  The New Premises Lease Term shall be extended for an additional term of sixty-five (65) months commencing on June 1, 2007 (the “ Extension Date ”) and terminating on October 31, 2012 (the “ Termination Date ”).

2.                                        New Premises .  Commencing on the Extension Date, the New Premises shall be deemed to contain approximately twenty-four thousand three hundred five (24,305) rentable square feet of space located on the first floor of the Building as depicted on the floor plan attached hereto as Exhibit A .

3.                                        Monthly Base Rent .  Effective on the Extension Date, Section 4 of the Lease is amended by deleting it in its entirety and inserting the following in lieu thereof:

Tenant shall pay to Landlord as Base Rent for the New Premises, accruing on and after the Extension Date and monthly thereafter in accordance with the terms of the Original Lease, as follows:

Tenant’s Pro Rata Share of Increased Operating Expenses as provided herein and such other charges as are required by the terms of this Lease to be paid by Tenant shall be referred to as “ Additional Rent .”  Landlord shall have the same rights as to the Additional Rent as it has in the payment of Base Rent.  The Base Rent, Additional Rent and all other amounts due to be paid under this Lease may be referred to herein collectively as “ Rent .”

The term “ Lease Year ” as used herein shall be each twelve (12) month period in the New Premises Lease Term commencing on June 1 of each calendar year during the New Premises Lease Term.

4.                                        Base Year Operating Expenses .

(a)                                   “ Base Operating Expenses ” shall mean an amount equal to the actual Operating Expenses for the Building Complex for the calendar year 2007.

(b)                                  Effective as of the Extension Date, Subparagraph 5.B. of the Third Amendment is hereby deleted and restated in its entirety as follows:

5.B.     From and after the Extension Date, Tenant shall pay to Landlord Tenant’s Prorata Share of any increases in Operating Expenses in excess of Base Operating Expenses (the “ Increased Operating Expenses ”) during each calendar year of the New Premises Lease Term and any extension thereof.  All amounts required to be paid by Tenant shall be paid within thirty (30) days following billing therefor by Landlord.  Beginning on January 1, 2008, and continuing each month thereafter during the New Premises Lease Term, or any extension thereof, Tenant shall pay to Landlord, at the same time as the Base Rent is paid, an amount equal to one-twelfth (1/12) of Landlord’s estimate (as determined by Landlord’s Accountants) of Tenant’s Prorata Share of the Increased Operating

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Expenses for the particular calendar year, with a final adjustment to be made between the parties at a later date for said calendar year.

(1)                                   As soon as practicable following the end of each calendar year during the New Premises Lease Term, or any extension thereof, Landlord shall submit to Tenant a statement prepared by a representative of Landlord setting forth the exact amount of Tenant’s Prorata Share of the Increased Operating Expenses for the calendar year just completed.  Beginning with said statement for the second full calendar year, it shall also set forth the difference, if any, between Tenant’s actual Prorata Share of the Increased Operating Expenses for such calendar year just completed and the estimated amount of Tenant’s Prorata Share of Increased Operating Expenses.  Each such statement shall also set forth the projected Increased Operating Expenses, if any, for the new calendar year and the corresponding increase in Tenant’s Prorata Share of the Increased Operating Expenses computed in accordance with the foregoing provisions; provided, however, in no event will the Rent to be paid by Tenant hereunder ever be less than the Base Rent as it is to be adjusted for such calendar year.  To the extent that Tenant’s Prorata Share of the Increased Operating Expenses for the period covered by such statement is different from the estimated amount upon which Tenant paid Rent during the calendar year just completed, Tenant shall pay to Landlord the difference in cash within thirty (30) days following receipt by Tenant of such statement from Landlord or receive a credit on the next month’s rental owing hereunder, as the case may be.  Until Tenant receives such statement, Tenant’s monthly Rent for the new calendar year shall continue to be paid at the rate paid for the particular calendar year just completed, but Tenant shall commence payment to Landlord of the monthly installments of Rent on the basis of such statement beginning on the first day of the month following the month in which Tenant receives such statement.  Moreover, Tenant shall pay to Landlord or deduct from the Rent, as the case may be, on the date required for the first payment of Rent, as adjusted, the difference, if any, between the monthly installments of Rent so adjusted for the new calendar year and the monthly installments of Rent actually paid during the new calendar year.

(2)                                   In addition to the above, if, during any particular calendar year, there is a change in the information on which Landlord’s Accountants based the estimate upon which Tenant is then paying its estimated Tenant’s Prorata Share of the Increased Operating Expenses so that such estimate furnished to Tenant is no longer accurate, Landlord shall be permitted to revise such estimate by notifying Tenant and there shall be such adjustments made in the monthly estimated Tenant’s Prorata Share of the Increased Operating Expenses on the first day of the month following the serving of such statement on Tenant as shall be necessary by either increasing or decreasing, as the case may be, the amount of Tenant’s Prorata Share of the Increased Operating Expenses then being paid by Tenant for the balance of the calendar year, as well as an appropriate adjustment in cash based upon the amount theretofore paid by Tenant during such particular calendar year pursuant to the prior estimate.

(3)                                   In the event the Rentable Area is not fully occupied during any particular calendar year, Landlord’s Accountants may adjust those Operating Expenses which are affected by the occupancy rates for the particular calendar

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year, or portion thereof, as the case may be, to reflect an occupancy of not less than ninety-five percent (95%) of such Rentable Area.  In the event the Rentable Area is not fully occupied during the calendar year 2007, which serves as the basis for the calculation of Base Operating Expenses, Landlord’s Accountants shall adjust those Operating Expenses which are affected by the occupancy rates for the calendar year 2007, or portion thereof, as the case may be, to reflect an occupancy of not less than ninety-five percent (95%) of such Rentable Area.

(4)                                   If the calendar year is not concurrent with the Lease Year, Landlord shall, at any time during the New Premises Lease Term, or any extension thereof, make all adjustments provided for in this Paragraph 5 with an appropriate proration for the Lease Year in which the New Premises Lease Term or any extension begins or ends.  In addition, Landlord may elect at any time during the New Premises Lease Term or any extensions thereof to make all adjustments provided for in this Paragraph 5 on a Fiscal Year basis with an appropriate proration for the calendar year in which such conversion is made and in which the term ends and all references in this Lease to calendar year shall thereafter be deemed to refer to “ Fiscal Year ”.

(5)                                   Landlord’s and Tenant’s responsibilities with respect to the Operating Expense adjustment described herein shall survive the expiration or early termination of this Lease.

(c)                                   Commencing as of the Extension Date, Tenant’s Prorata Share shall be deemed to be 20.207%.  The terms Tenant’s Porata Share and Tenant’s Pro Rata Share are used interchangeably in the Original Lease and shall carry the same meaning for all purposes hereof.

(d)                                  Nothing in the foregoing shall be deemed to modify any obligations of Tenant with respect to Operating Expenses for any period prior to the Extension Date.

5.                                        Tenant Finish Work .  Provisions regarding any remodeling of or tenant finish work to be completed in the  New Premises shall be as set forth below.  Except as provided below, Landlord shall have no obligation for completion or remodeling of the  New Premises and Tenant shall accept the Premises in its “as is” condition on the Extension Date:

(a)                                   Tenant Improvement Allowance .  Landlord agrees to pay Tenant a tenant improvement allowance of Two Hundred Ninety-One Thousand Six Hundred Sixty and No/100ths U.S. Dollars ($291,660.00) (the “ Tenant Improvement Allowance ”) as contribution toward the cost of tenant improvement work performed in the New Premises (collectively, the “ Tenant Finish Costs ” ) .  The Tenant Improvement Allowance shall be applied toward (i) the cost of tenant improvement work completed within the rentable area of the New Premises in

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accordance with the Construction Drawings (as defined below), excluding, however, any movable furniture, equipment and trade fixtures not physically attached to the Premises (collectively, the “ Tenant Work ”), and (ii) the cost of tenant improvement work completed within the rentable areas of the New Premises in accordance with Tenant’s IT Work (as defined below), as well as any soft costs, such as architectural, engineering, cabling and wiring costs incurred by Tenant in connection with Tenant’s Work or Tenant’s IT Work.  Landlord shall pay the costs of Tenant’s IT Work from the proceeds, if any, of the Tenant Improvement Allowance within thirty (30) days after receipt of invoices, reasonable supporting documentation, and lien waivers in such form as Landlord may reasonably require.  Any unused portion of the Tenant Improvement Allowance upon completion of Tenant’s Work and Tenant’s IT Work will not be refunded to Tenant and will not be available to Tenant as a credit against any obligations of Tenant under the Lease.   Landlord shall have the right to charge a construction management fee equal to two and one-half percent (2.5%) of the Tenant Finish Costs, which amount shall be deducted from the Tenant Improvement Allowance.

(b)                                  Design of Tenant’s Work .  Landlord and Tenant have approved those certain pricing plans for the Premises prepared by Intergroup Architects (the “ Architect ”) attached hereto as Exhibit C (collectively, the “ Pricing Plans “).  No changes may be made to the Pricing Plans without the written approval of Landlord and Tenant.

(c)                                   Construction Drawings .  Within fifteen (15) business days after the date of this Amendment, Landlord shall cause the Architect to prepare the construction drawings for Tenant’s Work in the New Premises (“ Construction Drawings ”) based upon the Pricing Plans and shall submit the Construction Drawings for Tenant’s approval.  Tenant shall either approve the Construction Drawings or submit, in writing, exceptions to the Construction Drawings to Landlord within five (5) business days of Tenant’s receipt of the Construction Drawings.  If Tenant does not respond to the Construction Drawings within such five (5) business day period, Tenant shall be deemed to have approved the Construction Drawings.  If Tenant submits exceptions to the Construction Drawings, Landlord shall respond to any of Tenant’s exceptions within five (5) business days of receipt thereof.  This procedure shall be repeated until both parties agree upon the final Construction Drawings.  After approval of the Construction Drawings no further changes may be made without the prior written approval of Landlord.

(d)                                  Construction of Tenant’s Work .  Once Landlord and Tenant agree on the final Construction Drawings, Landlord shall be responsible for the execution and administration of the construction of Tenant’s Work, which shall be done in accordance with the following provisions:

(i)                                      Contractor Selection .  Within fifteen (15) business days after the date Landlord and Tenant agree upon the final Construction Drawings, Tenant’s Work shall be competitively bid to three (3) general contractors, and Tenant shall be provided a reasonable opportunity to provide input into the bid and bid review process.  Landlo