<PAGE>

                                                                   EXHIBIT 10.18

                               LEASE AMENDMENT II

     The parties to this Amendment are Westlake Center Associates Limited
Partnership, a Washington limited partnership, hereinafter called "Lessor", and
PRIMUS KNOWLEDGE SOLUTIONS, INC. formerly doing business as PRIMUS
COMMUNICATIONS, formerly known as SYMBOLOGIC CORPORATION hereinafter called
"Lessee."

     Whereas the parties hereto have heretofore entered into a Lease executed
under the date of July 28, 1995, in and to 8,065 square feet on the Eighteenth
Floor and 16,660 square feet on the Nineteenth Floor, for a combined total of
24,725 square feet, known as Suite 1900, and further amended by a Lease
Amendment executed under the date of January 27, 1999 in and to 11,686 square
feet on the Eighteenth Floor and 16,660 square feet on the Nineteenth Floor, for
a combined total of 28,346 square feet, known as Suite 1900 the following
described premises in the City of Seattle, County of King, State of Washington,
to wit:

          Lots 1, 2, 3, 7, 8, 9, 10, 11 and 12, Block 1, ADDITION TO
          THE TOWN OF SEATTLE as laid off by the Heirs of SARAH A.
          BELL, deceased (commonly known as Heirs of SARAH A. BELL'S
          ADDITION TO THE CITY OF SEATTLE) according to the plat
          recorded in Volume 1 of Plats, page 103, in King County, Washington;

          EXCEPT the Southwesterly 12 feet of said Lots 1, 2 and 3
          condemned by the City of Seattle in King County Superior
          Court Cause No. 52280, for the widening of Fourth Avenue, as
          provided by Ordinance 13776 of said City; and

          EXCEPT the Southeasterly 7 feet of said Lots 1 and 12
          condemned by the City of Seattle, in King County Superior
          Court Cause No. 57057 for the widening of Pine Street as
          provided by Ordinance 14500 of said City;

          TOGETHER WITH all of the vacated alley lying within said
          Block 1.  The Property is generally bounded by Fifth Street
          on the east, Pine Street on the south, Fourth Avenue on the
          west, and Olive Way on the north.

     Whereas the parties desire to renew the Lease and expand the Premises by an
additional 4,974 square feet of Net Rentable Area on the 18th Floor and 16,430
square feet of Net Rentable Area on the 10th Floor for a total expansion of
21,404 square feet of Net Rentable Area of said Lease.   Now, therefore, for
good and valuable consideration, the receipt and adequacy of which is hereby
acknowledged, the parties agree to amend the Lease, effective June 1, 2000, as
follows:

     1.   Section 1(b), Premises, is amended to show that the square footage of
          the Premises has been revised from 28,346 S.F. to 49,750 S.F.
          Therefore, Section 1(b), Premises, is amended to read as follows:

          "Premises:  Consisting of 16,660 square feet of net rentable area on
          the 19th floor 16,660 square feet of net rentable area on the 18th
          floor and 16,430 square feet of net rentable area on the 10th floor
          for a combined total of 49,750 square feet of net rentable area of the
          building, as outlined on the floor plan attached hereto as Exhibit C-
          2, including tenant improvements, if any, as described in Exhibit A-
          2."

          Section 1(b), Premises, is further amended to allow Lessor to re-
          measure the Premises during the term of this Amendment using the
          current the BOMA standard for measurement provided, however, that said
          measurement shall not alter Lessee's Basic Monthly Rent as stipulated
          in this Lease Amendment II.

     2.   Section 1(c), Floor Areas, is amended to show that the agreed rentable
          area of the Premises has been revised from 28,346 square feet to
          49,750 square feet. Therefore, Section 1(c) is amended to read as
          follows:

          "The agreed net rentable area of the Premises is 49,750 square feet
          and of the building is 335,850 square feet."

                                       1
<PAGE>

     3.   Section 1(e), Commencement Date, is amended to show the terms for the
          Lease Renewal and Lease Expansion spaces commence- as follows:

<TABLE>
<CAPTION>
Suite     Square Footage     Commencement Date      Renewal/Expansion
-----     --------------     -----------------      -----------------

<S>       <C>               <C>                    <C>
1815       1,475             June 1, 2000           Expansion
1000      16,430             September 1, 2000      Expansion
1800       3,499             November 1, 2000       Expansion
1801      11,686             November 1, 2000       Renewal
1900      16,660             November 1, 2000       Renewal
          ------
TOTAL     49,750
</TABLE>

     4.   Section 1(f), Expiration Date, for the existing 28,346 square feet is
          amended by deleting October 31, 2000 and inserting October 31, 2005 as
          the new Lease expiration date. The expiration date for the 21,404
          square feet of expansion space shall be co-terminus with the 28,346
          square feet. Therefore, the expiration date for the 49,750 square feet
          shall be October 31, 2005.

     5.   Section 1(j), Exhibits, is amended to include the attached Exhibit A-
          2, Tenant Improvements for Expansion and Existing Space, replacing the
          existing Exhibits A and A-1.

     6.   Section 1(j), Exhibits, is amended to include the attached Exhibit C-
          2, The Premises - Existing and Expansion, replacing the existing
          Exhibits C and C-1.

     7.   Attachment II, Special Provision #2, Monthly Rent, shall be amended to
          include an additional 21,404 net rentable square feet and include a
          rental rate structure for the Renewal Term.  Therefore, the rental
          rate structure in Attachment II, Special Provision #2, of the Lease
          Agreement, shall be adjusted to reflect increases to the Net Rentable
          Area, Term Extension and Monthly Rent upon substantial completion of
          the tenant improvements as outlined in the attached Exhibit A-2 as
          follows:
<TABLE>
<CAPTION>

                                    Square             Basic        Basic
                                    Feet of            Annual       Monthly
Period                              NRA       Floor    Rental Rate  Rent
----------------------------------  ------    -------  -----------  -----------
<S>                                 <C>       <C>       <C>       <C>

06/01/00 - 10/31/02                  1,475     18th       $33.00    $  4,056.25
09/01/00 - 10/31/02                 16,430     10th       $30.00    $ 41,075.00
11/01/00 - 10/31/02                 15,185     18th       $33.00    $ 41,758.75
11/01/00 - 10/31/02                 16,660     19th       $33.00    $ 45,815.00
                                    ------                          -----------
TOTAL:                              49,750                          $132,705.00


                                    Square             Basic        Basic
                                    Feet of            Annual       Monthly
Period                              NRA       Floor    Rental Rate  Rent
----------------------------------  ------    -------  -----------  -----------

11/01/02 - 10/31/03                 16,430     10th      $31.00     $ 42,444.17
11/01/02 - 10/31/03                 16,660     18th      $34.00     $ 47,203.33
11/01/02 - 10/31/03                 16,660     19th      $34.00     $ 47,203.33
                                    ------                          -----------
TOTAL:                              49,750                          $136,850.83


                                    Square             Basic        Basic
                                    Feet of            Annual       Monthly
Period                              NRA       Floor    Rental Rate  Rent
----------------------------------  ------    -------  -----------  -----------

11/01/03 - 10/31/05                 16,430     10th      $32.00     $ 43,813.33
11/01/03 - 10/31/05                 16,660     18th      $35.00     $ 48,591.67
11/01/03 - 10/31/05                 16,660     19th      $35.00     $ 48,591.67
                                    ------                          -----------
TOTAL:                              49,750                          $140,996.67
</TABLE>

                                       2
<PAGE>

     8.   Attachment I, Special Provision #23, Brokers, is deleted in its
          entirety and replaced with a new Special Provision #23, Brokerage
          Fees, as follows:

          "Lessor shall pay Colliers Macaulay Nicolls International a brokerage
          fee as follows:

                                            Brokerage Fee        Brokerage
          Suite        Square Footage      Per Square Foot      Commission
          -----        --------------      ---------------      -----------
          1815             1,475                $3.50           $  5,162.50
          1000            16,430                $3.50           $ 57,505.00
          1800             3,499                $3.50           $ 12,246.50
          1801            11,686                $1.75           $ 20,450.50
          1900            16,660                $1.75           $ 29,155.00
          TOTAL:          49,750                                $124,519.50

          Lessee warrants that it has had no dealings with any real estate
          broker or agent in connection with the negotiation of this Lease
          Amendment II excepting only Colliers Macaulay Nicolls International
          and it knows of no other real estate broker or agent who is entitled
          to a commission in connection with this Lease Amendment II."

     9.   Attachment II, Special Provision #6, Parking, is hereby amended to
          show that Lessee shall have the option to enter into parking contracts
          for up to seventeen (17) additional parking contracts for a total of
          forty (40) parking contracts. Said contracts shall be available upon
          commencement of expansion spaces as follows:

          Commencement Date of Expansion             Parking Contracts Available
          ------------------------------             ---------------------------
          June 1, 2000                               One (1)
          September 1, 2000                          Thirteen (13)
          November 1, 2000                           Three (3)

          Tenants may use these parking contracts throughout the term of this
          Agreement at the prevailing rate charged by the operator of the garage
          plus all applicable Washington State and local taxes.  Additional
          parking may be taken with any future expansion in the building at a
          ratio of 1/1,200 square feet of Net Rentable Area at the prevailing
          rate charged by the operator of the garage plus all applicable
          Washington State and local taxes.

     10.  Attachment II, Special Provisions, Number 3, Lease Renewal Options is
          hereby deleted in its entirety.

     11.  Attachment II, Special Provisions, Number 4, Expansion Option/Right of
          First Offer is hereby deleted in its entirety and replaced with the
          following:

          "Subject to the rights of existing tenants as stated below, Lessee
          shall have a one (1) time Right of First Offer to lease the following
          spaces as summarized immediately following:

                  Net             Current Lease     Pre-Existing
          Suite   Rentable Area   Expiration Date   Tenant Options
          -----   -------------   ---------------   --------------------------
          610        12,301          01/31/01       Existing tenant has one
                                                    renewal right for three
                                                    additional years

          1100        5,242          12/31/02       Right of First Refusal To
                                                    Expand into Suite 1102
                                                    During Current Term.
                                                    Tenant has no Renewal
                                                    Rights.

          1101        9,903          10/31/02       None.

          1102        1,285          03/31/00       None.


                                       3
<PAGE>

          Lessee shall exercise its Right of First Offer by written notice to
          Lessor within ten (10) days following receipt of written notice from
          Lessor stating that the Additional Premises is available for lease and
          the market rental rate and terms under which the space is available.
          Lessor may issue its written notice up to nine (9) months in advance
          of the date said space is due to come available.

          In the event that Lessee exercises the right granted herein, Lessor
          and Lessee shall enter into an amendment to the Lease to incorporate
          the Additional Premises and to make necessary adjustments to the
          Annual Basic Rent and similarly affected provisions of the Lease.  In
          the event Lessee declines to exercise its right as above provided for,
          or fails to deliver notice thereof within the time period stipulated
          above, or fails to execute the requisite amendment to this Lease, this
          right of first offer shall lapse and be of no further force and
          effect.

          The foregoing right of first offer shall not be severed from the Lease
          or separately sold, assigned, or transferred and shall be subject to
          the following additional conditions, namely: (a) that, at the time
          that Lessee exercises this right of first offer for any additional
          space, Lessee shall not be in default of any term, covenant, or
          obligation of this Lease; (b) that, at the time Lessee exercises this
          right of first offer, Lessee shall be in occupancy and possession of
          the Premises; and (c) the pre-existing rights of any other tenant to
          the Additional Premises have not been exercised."

     12.  Section 9(a.9.) "Base Year" is amended such that, upon the
          Commencement Dates of the spaces identified in paragraph #3 above, the
          Base Year shall be converted from calendar year 1996 to a new base
          year of 2000. Therefore, Section 9 (a.9.) shall read as follows:

          " 'Base Year' shall mean calendar year 2000 upon the Commencement
          Dates as stated in paragraph #3 above."

     13.  Section 6, Security Deposit is hereby amended by adding the following:

          "Upon execution of this Amendment II, Lessee shall deposit with Lessor
          either (a) a cash amount required to bring its total security deposit
          to a sum of $140,996.67 or (b), a Letter of Credit, in a form as
          mutually agreed, in the amount of Two Hundred and Fifty Thousand
          Dollars ($250,000.00).  Said funds shall be considered as an
          additional Security Deposit toward the performance of Lessee's
          obligations under the Lease Agreement and as further amended under
          this agreement, Lease Amendment II."


     Except as herein amended, said Lease shall remain in full force and effect
in accordance with all of its terms and provisions.

     IN WITNESS HEREOF, the parties have hereinto set their hands and seals this
11th day of February, 2000.


LESSOR:                             LESSEE:

WESTLAKE CENTER ASSOCIATES          PRIMUS KNOWLEDGE SOLUTIONS,
LIMITED PARTNERSHIP                 INC.

By: /s/  J. Patrick Done             By: /s/  Elizabeth J. Huebner

Its: VP/Grp Dir.                    Its: EVP-CFO


                                       4
<PAGE>

ACKNOWLEDGMENT OF LESSOR


STATE OF Nevada    )
                   ) ss.
COUNTY OF Clark    )

     On this 11th day of February, 2000, before me, the undersigned, a Notary
Public in and for the State of Nevada, duly commissioned and sworn, personally
appeared J. Patrick Done, to me known to be the VP/Group Director, that executed
the within and foregoing instrument, and acknowledged said instrument to be the
free and voluntary act and deed of said partnership, for the uses and purposes
therein mentioned and on oath stated that he/she was authorized to execute said
instrument.

     WITNESS my hand and official seal hereto affixed the day and year in this
certificate above written.

                                              /s/ Gail M. Denney McGowan
                  [Notary Seal                Notary Public in and for the
                  affixed here]               State of Nevada residing at
                                              Las Vegas
                                              My commission expires 11/12/2001
                                              Print Name Gail M. Denney McGowan


                            ACKNOWLEDGMENT OF LESSEE

(Corporate)

STATE OF WASHINGTON )
                    ) ss.
COUNTY OF KING      )

     On this 9th day of February, 2000, before me personally appeared Elizabeth
J. Huebner to me known to be the Exec. VP & CFO of the corporation that executed
the within and foregoing lease, and acknowledged the said instrument to be the
free and voluntary act and deed of said corporation, for the uses and purposes
therein mentioned, and on oath stated that they were authorized to execute said
instrument and that the seal affixed is the corporate seal of said corporation.

     IN WITNESS WHEREOF, I have hereunto set my hand and affixed my official
seal the day and year first above written.

                                    /s/ Mary Robertson
                                    Notary Public in and for the State of
                  [Notary Seal      Washington residing at Bothell
                  affixed here]     My commission expires 12/25/2000
                                    Print Name Mary Robertson

                                       5
<PAGE>

                                  EXHIBIT A-2
                       TENANT IMPROVEMENTS FOR EXPANSION
                                      AND
                                 EXISTING SPACE


     Effective upon the execution of this Lease Amendment II by Lessor and
     Lessee, Lessor shall make available to Lessee, under terms as stipulated
     below, a Tenant Improvement Allowance for physical improvements to the
     entire existing and expanded Premises of up to, but not to exceed three
     hundred thirty thousand nine hundred dollars ($330,900).

     Said Tenant Improvement Allowance is to be used by December 31, 2001 at
     which time all remaining sums, if any, not used by Lessee shall revert back
     to Lessor.  All sums for work done in the Premises over the Tenant
     Improvement Allowances as stated above shall be the sole responsibility of
     Lessee.

     Tenant Improvements shall mean only physical improvements made to the
     Premises and shall exclude trade fixtures, furniture, movable furniture
     partitions, storage shelving, installing telephones, telephone and cables,
     computer/data cable, fine finish items such as wall coverings, etc., and
     similar items.

     Lessee shall use Lessor or other Lessor approved contractor for its tenant
     improvement work.  Lessee understands it will be required to use Lessor's
     sub-contractors for any, and all, HVAC, Mechanical and electrical work to
     be completed in the Premises.

     In the event that Lessee contracts with an approved third party contractor
     for Tenant Improvements, Lessee may submit a request for reimbursement of
     said expenses based on actual invoices from Lessee's contractors and or
     vendors along with lien releases for the work performed in the Premises.
     Lessor shall reimburse Lessee within thirty (30) days after receipt of the
     reimbursement request, but not prior to the commencement of the renewal and
     expansion of the Premises or the date that the Tenant Improvement Allowance
     has been made available as stated above.  In the event that Lessee employs
     a third party contractor to provide construction services for tenant
     improvements, Lessee shall pay to Lessor a fee equal to ten percent (10%)
     of the total tenant improvement allowance to compensate Lessor for costs
     associated with the coordination, review and monitoring of Lessee's
     contractor during the construction period.

     All improvements made to the Premises are subject to Section 11,
     Alterations by Lessee, of the Lease Agreement and must be approved in
     writing by Lessor prior to the commencement of any improvements.

                                       6
<PAGE>

                                  EXHIBIT C-2
                     THE PREMISES - EXISTING AND EXPANSION
                                  Page 1 of 3



                              [Primus Suite 1000
                                   16,430 NRA
                    Expansion Period:  01-Sep-00 - 31-Oct-05

                                 DIAGRAM HERE]



                         WESTLAKE CENTER OFFICE TOWER
                                  TENTH FLOOR

                            KOEHLER McFADYEN & Co.
                                 206-682-2680
                                 STEVE KOEHLER

                                       7
<PAGE>

                                  EXHIBIT C-2
                     THE PREMISES - EXISTING AND EXPANSION
                                  Page 2 of 3



                              [Primus Suite 1801
                                  11,686 NRA
                       Existing Lease Expires: 31-Oct-00
                     Renewal Period: 01-Nov-00 - 31-Oct-05


                               Primus Suite 1800
                                   3,499 NRA
                    Expansion Period:  01-Nov-00 - 31-Oct-05


                               Primus Suite 1815
                                   1,475 NRA
                    Expansion Period:  01-Jun-00 - 31-Oct-05

                                 DIAGRAM HERE]



                         WESTLAKE CENTER OFFICE TOWER
                               EIGHTEENTH FLOOR

                            KOEHLER McFADYEN & Co.
                                 206-682-2680
                                 STEVE KOEHLER

                                       8
<PAGE>

                                  EXHIBIT C-2
                     THE PREMISES - EXISTING AND EXPANSION
                                  Page 3 of 3



                              [Primus Suite 1900
                                   16,660 NRA
                       EXISTING LEASE EXPIRES: 31-OCT-00
                     Renewal Period:  01-Nov-00 - 31-Oct-05

                                 DIAGRAM HERE]



                         WESTLAKE CENTER OFFICE TOWER
                               NINETEENTH FLOOR

                            KOEHLER McFADYEN & Co.
                                 206-682-2680
                                 STEVE KOEHLER

                                       9