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                                                             EXHIBIT 10.23


                                    SUBLEASE



         THIS SUBLEASE made as of the ____day of February, 1997 between National
Medical Care, Inc. d/b/a Fresenius Medical Care - North America, a Delaware
corporation having a usual place of business at 95 Hayden Avenue, Lexington,
Massachusetts 02173 (hereinafter called "Sublandlord"), and Geotel
Communications Corporation, a Delaware corporation having a place of business at
25 Porter Drive, Littleton, Massachusetts 01460 (hereinafter called
"Subtenant").

         WHEREAS, by a lease dated as of October 23,1995, (hereinafter called
the "Main Lease"), a copy of which is attached hereto as Exhibit A with the
rental amounts deleted, Cross Point Limited Partnership (hereinafter called
"Landlord"), leased to Sublandlord approximately 142,753 square feet of space in
the building located at 900 Chelmsford Street, Lowell, Massachusetts (the "Main
Premises") upon the terms, covenants and conditions therein contained; and

         WHEREAS, Sublandlord has agreed to sublease to Subtenant a portion of
the premises demised under the Main Lease comprised of approximately 31,770
square feet in the location set forth in Exhibit B attached hereto (the
"Subleased Premises"), on the terms stated herein.

         NOW, THEREFORE, for good and valuable consideration, the receipt and
sufficiency of which is hereby acknowledged, the parties hereto agree as
follows:

1.       Sublandlord hereby leases to Subtenant, and Subtenant hereby takes and
leases from Sublandlord, the Subleased Premises; TO HAVE AND TO HOLD the
Subleased Premises for a term (the "Term") which commences on the date Landlord
gives consent to this Sublease (the "Commencement Date") and ends on December
31, 2006. The foregoing notwithstanding, Subtenant shall have the right to
terminate this Sublease at the end of the Seventy-second (72nd) month of the
Term (six years), provided Subtenant gives Sublandlord at least twelve (12)
months prior written notice.

2.       The Subleased Premises shall be delivered to Subtenant in their "as is"
condition on the Commencement Date. The existing partitions, lights, doors,
ceiling tiles and any and all other building materials belonging to Sublandlord
located within the Premises on the Commencement Date shall become the permanent
property of Subtenant at no additional cost to Subtenant. Subtenant shall have
the same rights to make initial improvements to the Subleased Premises (the
"Initial Improvements") as set forth in Section 4 of the Main Lease, except that
Sublandlord shall have the same rights to review and approve such work as the
rights granted to Landlord under Section 4 of the Main Lease, and that such
rights of the Sublandlord shall be in addition to the rights of the Landlord
under Section 4 of the Main Lease. The plans and specifications for the Initial
Improvements are attached hereto as Exhibit C and made a part hereof. All
Initial Improvements and any and all other improvements to the Subleased
Premises will be made at the expense of Subtenant and in accordance with all
applicable local, state and federal laws, rules and regulations, including the
Americans With Disabilities Act.

3.       3.1. Sublandlord grants to Subtenant, to share in common with
Sublandlord, all of Sublandlord's rights, benefits, and interests (except as
otherwise provided for herein) and Subtenant 
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agrees to accept from Sublandlord and hereby assumes all of Sublandlord's
obligations under the Main Lease. Subtenant hereby expressly agrees and
covenants to Landlord and to Sublandlord, that Subtenant assumes, is bound by
and shall faithfully perform, all of the obligations of Sublandlord as tenant
under the Main Lease with respect to the Subleased Premises, except as otherwise
expressly provided for herein. It is further agreed that the relationship
between, and the rights of, Subtenant and Sublandlord shall, except as expressly
provided for herein, be governed by the Main Lease as if they were tenant and
landlord, respectively, under the Main Lease, and that Sublandlord specifically
shall have all rights granted to Landlord under the Main Lease with respect to
enforcement of the provisions of this Sublease and the termination hereof;
provided, however, that Sublandlord shall not be deemed to guarantee performance
by Landlord of its obligations under the Main Lease, and Sublandlord shall have
no liability to Subtenant for any default in this Sublease caused by the default
or any other act of Landlord under the Main Lease. However, if Landlord shall
default in any of its obligations to Sublandlord under the Main Lease,
Sublandlord shall cooperate with Subtenant, upon request by Subtenant, in
enforcing Sublandlord's rights against Landlord under the Main Lease. Any
out-of-pocket costs associated with enforcing such rights shall be incurred
solely by Subtenant unless there is some mutual benefit between Subtenant and
Sublandlord in enforcing Landlord's obligations under the Main Lease, in which
case such costs shall be equitably shared between Subtenant and Sublandlord. It
is further agreed that performance by Sublandlord shall be conditional upon the
performance by Landlord of its obligations under the Main Lease. Sublandlord
shall be entitled to exercise any of the remedies set forth in Section 16 of the
Main Lease upon the occurrence of any of the defaults set forth therein.

         3.2. The following provisions of the Main Lease are excluded from the
terms hereof, and Subtenant shall have no rights thereunder: items 6,7,8,9, 10
and 15 of the Lease Schedule except as specifically provided for herein;
subsection (c) of Section 24 (Parking); Section 26 (First Option to Extend);
Section 27 (Second Option to Extend); Section 28 (Right of First Offer to
Lease); Section 29 (First Expansion Option); and Section 30 (Second Expansion
Option). The second and third sentences of Section 3 are excluded such that
Subtenant shall not have the right to operate or maintain a health club or
fitness room within the Subleased Premises. The second paragraph of Section 32
(Signage) is excluded such that Subtenant shall have no right to exterior
signage.

         3.3 The following references to numbers of days in the Main Lease are
changed as follows with respect to the relationship between Sublandlord and
Subtenant:

         (i)   subsection 2.C (v) - 90 days to inspect Landlord's accounting
               records relative to Taxes and Expenses is reduced to 75 days;

         (ii)  subsection 2.C.(ix) - 10 working days to pay certain amounts
               after receipt of notice that amounts are due is reduced to 7
               working days;

         (iii) subsection 5.E - 30 days of utility outage after notice is
               received by Landlord is increased to 32 days;

         (iv)  subsection 10.A -15 days prior to commencement and expiration of
               insurance policies to deliver certificates of insurance to
               Landlord is increased to 20 days;


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         (v)    subsection 12.F - 60 days after casualty for Landlord to notify
                Tenant of its decision to restore is increased to 65 days, and
                15 days for Tenant to inform Landlord of exercise of a certain
                termination right is reduced to 7 days;

         (vi)   Section 13 - 30 days after notice of condemnation to give notice
                of exercise of right to terminate is reduced to 20 days in the
                case of Tenant's notice to Landlord and increased to 35 days in
                the case of Landlord's notice to Tenant;

         (vii)  subsection 16.A.(ii) - all references regarding cure that
                prescribe 30 days are reduced to 25 days, and all references
                prescribing 20 days are reduced to 15 days;

         (viii) subsection 16.A.(iii) - 60 days for discharge of levy or
                attachment is reduced to 45 days;

         (ix)   subsection 16.A.(v) -10 days to deliver certificates is reduced
                to 7 days;

         (x)    subsection 16.A.(vi) -10 days to deliver instruments is reduced
                to 7 days;

         (xi)   Section 18 -10 days' prior request to deliver certificates is
                reduced to 7 days;

         (xii)  Section 33 - 30 day cure periods and payment periods are
                increased to 40 days.

         3.4. Subtenant's parking rights shall be comprised of its pro rata
share of Sublandlord's allocable share of vehicular parking spaces in the
Parking Lot and the Parking Garage, if any, as set forth in subsections 24.A.
and 24.B. of the Main Lease. Subtenant shall not be entitled to any credit
against the Rent due under this Sublease notwithstanding any provision of
subsection 24.B.

         3.5. Notwithstanding anything herein to the contrary, Subtenant's
rights of access to Tower 2 Roof shall be limited as follows: All rights of
Subtenant pursuant to Section 35 of the Main Lease are subject to Landlord's
consent, and shall be limited by Sublandlord as reasonably appropriate to enable
Sublandlord's other subtenants in the Main Premises, existing now or hereafter,
to have access to the Tower 2 roof in accordance with Section 35 of the Main
Lease. To the extent practicable, Sublessor's grant of Subtenant's rights under
this Section 3.8 shall be based on Subtenant's proportionate share of the Main
Premises.

4.       Subtenant covenants and agrees with Sublandlord to pay to Sublandlord
as rent during the term hereof as follows:

         4.1. Base Rent shall be paid in advance on the first day of each month
of this Sublease beginning on the Occupancy Date and continuing through the
remaining term of this Sublease. For purposes of this Sublease, the Occupancy
Date is defined as the date Subtenant substantially completes its Initial
Improvements or occupies the Subleased Premises, whichever occurs first.
However, in the event of unreasonable delays caused by Subtenant which result in
Subtenant being unable to occupy the Subleased Premises on or before May 15,
1997, the Commencement Date will be May 15, 1997. Base Rent will be as follows:


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<TABLE>
<S>                                                            <C>       
               Occupancy Date through March 31, 2002:          $25,151.25
               April 1, 2002 through December 31, 2006:        $30,446.25
</TABLE>

         4.2 All Adjustment Rent as defined in Section 3 of the Main lease and
all other additional rent including, without limitation, operating costs, taxes,
expenses, charges and fees required by the Main Lease allocable to the Subleased
Premises during the term on a pro rata square footage basis, shall accrue and be
payable by Subtenant beginning on the Occupancy Date and continuing for the
remaining term of this Sublease. However, the cost of electrical power and other
utilities supplied to the Subleased Premises shall accrue and be payable by
Subtenant beginning on the Commencement Date and continuing for the entire term
of this Subleases. There will be a pro rata adjustment to the Base Rent and all
Adjustment and additional rent for the month in which either the Commencement
Date or the Occupancy Date occurs, if such date is a day other than the first
day of the month.

         4.3 After the end of each calendar year during the Term after the Base
Year, Sublandlord shall deliver to Subtenant a written statement prepared in
accordance with generally accepted accounting principles showing for the
calendar year just ended (i) the amount of building expenses, as defined in the
Main Lease, (ii) the excess of building expenses above the Base Year amount, and
(iii) the aggregate amount previously paid by Subtenant for the calendar year.

         If the statement shows the excess for the calendar year just ended
exceeded the amount paid by Subtenant on account thereof, Subtenant shall pay
the amount of such difference to Sublandlord on the later of thirty (30) days or
when the next Base Rent payment is due. If the statement shows that the amount
paid by Subtenant hereof on account of Adjustment Rent for the calendar year
just ended exceeded the excess of Adjustment Rent above the Base Year amount,
such amount shall be credited by Sublandlord against the monthly installments of
Adjustment Rent next due. Should such excess occur during the last month of the
term, Sublandlord shall refund such excess to Subtenant. Sublandlord shall
provide Subtenant appropriate documentation that itemizes all the building
expenses for the previous calendar year, upon Subtenants request.

         4.4. All payments under this Section 4 shall be made by Subtenant
without any setoff or deduction whatsoever, in lawful money of the United
States. Such payments shall be paid to Sublandlord at Sublandlord's office
hereinabove set forth or at such other place or to such other party or parties
as Sublandlord may from time to tome designate by notice to Subtenant. Subtenant
shall pay Sublandlord interest at the default rate set forth in the Main Lease
on any payments required by this Section 4 which are not made by Subtenant when
due.

5.       Subtenant covenants and agrees to use the Subleased Premises for office
purposes consistent with a first-class office building and for no other purpose.
Subject to Landlord's consent and to the terms of the Main Lease, Subtenant may
use the Subleased Premises for the following: general and executive offices;
sales, service and display of computers and related equipment and supplies;
computer operations; demonstration of Subtenant's products and customer/employee
training in the use thereof; receiving, shipping and storage of computers,
equipment and supplies associated with the foregoing; computer related research
and development; prototype assembly; and shipping and receiving packages and
equipment. To the extent permitted by Landlord, Subtenant shall be 


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permitted to ship and receive packages and equipment by and between the loading
docks and other areas within the Main Premises.

6.       Sublandlord warrants and represents that it has no knowledge of any
default by itself or by landlord under the Main Lease; that the Main Lease is in
full force and effect and that Sublandlord has a good right to sublease its
interest in the same provided Sublandlord obtains the consent of Landlord in
accordance with Subsection 14(A) of the Main Lease and obtains the consent of
Landlord's lender or lenders who may have approval rights in connection with
this Sublease; and that Sublandlord has done nothing to defeat or impair this
Sublease. Sublandlord further warrants and covenants that Subtenant, upon
performance of Subtenant's obligations hereunder and subject to the provisions
hereof, shall for the term hereof, succeed to all rights of Sublandlord under
the Main Lease with respect to the Subleased Premises and will have quiet
possession of the Subleased Premises unless the Main Lease be terminated for any
reason; provided, however, that this Sublease shall be in all respects subject
to the Main Lease and if the Main Lease shall terminate during the term hereof
for any reason, this Sublease shall terminate upon such termination with the
same force and effect as if such termination date had been named herein as the
termination date hereof; and if any provisions of this Sublease shall be in
violation of the provisions of the Main Lease, the provisions of the Main Lease
shall be deemed to limit the provisions hereof. It is expressly understood and
agreed, however, that nothing stated in this Section 6 shall be deemed to confer
upon Subtenant any greater rights than are set forth herein nor limit any of the
Subtenant's obligations hereunder.

7.       Subtenant agrees to do nothing which will subject the Main Lease to
termination by Landlord under the provisions of the Main Lease, and it is
further agreed that if Subtenant is in default of the provisions of the Main
Lease, Sublandlord may, after seven (7) days written notice to Subtenant or
immediately in the event of an emergency, but need not, cure said default
specifically on behalf of and for the account of Subtenant, in which case all
costs, damages, and expenses incurred by Sublandlord in connection therewith
shall be paid to Sublandlord immediately upon its demand as additional rent
hereunder. By curing Subtenant's default on its behalf and account, as
aforesaid, Sublandlord shall not be deemed to have waived any of its rights nor
to have released Subtenant from any of its obligations under this Sublease. It
is agreed, however, that Sublandlord may cure said default on its own account to
preserve its interest in the Main Leases and may terminate this Sublease
pursuant to the terms hereof by reason of said default by Subtenant, if
Subtenant does not pay as additional rent to Sublandlord all costs, damages and
expenses incurred by Sublandlord in connection with such cure within the
applicable grace period provided for in the Main Lease, as amended by Section 3
above. In the event of such termination, Sublandlord shall be entitled to all
remedies and damages provided for Landlord in the Main Lease, or as otherwise
provided by law. In the event that the Sublease is terminated by reason of
Subtenant's default, Subtenant shall indemnify and hold Sublandlord harmless
from such damages as Sublandlord may become liable to pay under the Main Lease
with respect to the Sublease Premises resulting from such default, plus all
other expenses and costs related thereto, including without limitation
attorneys' fees.

8.       Subtenant shall be permitted to install its own signage within the
Building, subject to approval by the Landlord.

9.       Subtenant shall have the right to sublet or assign this Sublease
subject to the consent of the Landlord and Landlord's right to recapture under
the terms and conditions of the Main Lease and 


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also subject to Sublandlord's independent consent, such consent by Sublandlord
not to be unreasonably withheld.

10.      Subtenant shall maintain with respect to the Subleased Premises the
insurance required by the Main Lease to be taken out by the tenant thereunder,
which insurance shall name Landlord and Sublandlord as additional insureds, all
in accordance with said sections of the Main Lease.

11.      At the expiration or earlier termination of this Lease, Subtenant shall
surrender and yield up the Subleased Premises in the condition required under
Section 15 of the Main Lease, and shall remove all equipment and trade fixtures
of Subtenant therefrom. If Subtenant shall remain in possession of the Subleased
Premises after the expiration or earlier termination of this Sublease without
any express agreement as to such holding over, Subtenant shall be liable to
Sublandlord in accordance with the Main Lease.

12.      Subtenant further agrees to indemnify and hold Sublandlord harmless
from any claim of Landlord under the Main Lease, and against any claim for
injury to persons, including death, and for property damages, arising out of the
occupancy and use of the Subleased Premises by Subtenant its officers, agents,
employees or invitees.

13.      Subtenant shall deposit with Sublandlord upon Subtenant's execution
hereof a security deposit in the amount of $37,726.88 (the "Security Deposit")
as security for Subtenant's faithful performance of Subtenant's obligations
under this Sublease. Subtenant shall pay an additional security deposit of
$5,295.00 on or before the first day of the 61st month of the term of this
Sublease, whereupon the Base Rent increases proportionally. If Subtenant fails
to pay Base Rent or other rent or charges due hereunder, or otherwise defaults
under this Sublease, Sublandlord may use, apply or retain all or any portion of
said Security Deposit for the payment of any amount due Sublandlord or to
reimburse or compensate Sublandlord for any liability, cost, expense, loss or
damage (including attorneys' fees) which Sublandlord may suffer or incur by
reason thereof. If Sublandlord uses or applies all or any portion of said
Security Deposit, Subtenant shall within ten (10) days after written request
therefore deposit monies with Sublandlord sufficient to restore said Security
Deposit to the full amount required by this Sublease. Sublandlord shall not be
required to keep all or any part of the Security Deposit separate from its
general accounts. Sublandlord shall, at the expiration or earlier termination
for the term hereof and after Subtenant has vacated the Premises, return to
Subtenant (or, at Sublandlord's option, to the last assignee, if any, of
Subtenant's interest herein), that portion of the Security Deposit not used or
applied by Sublandlord unless otherwise expressly agreed in writing by
Sublandlord, no part of the Security Deposit shall be considered to be held in
trust, to bear interest or other increment for its use, or to be prepayment for
any monies to be paid by Subtenant under this Sublease.

14.      All notices required to be given under this Sublease shall be sent by
prepaid registered or certified mail, return receipt requested, to Sublandlord,
to the attention of the Corporate Law Department, at the addresses above stated,
and to Subtenant at the Sublease Premises, unless in either case a different
address is specified by either party to the other in writing by prepaid
registered or certified mail, return receipt requested. Any such notices shall
be deemed to have been given when deposited with the U.S. Mail.


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15.      Sublandlord hereby reserves the right to enter onto the Subleased
Premises, from time to time at reasonable times and upon prior notice to
Subtenant which is reasonable under the circumstances, to ascertain whether
Subtenant is in compliance with the provisions of this Sublease.

16.      Upon written request by Subtenant, Sublandlord agrees to use reasonable
efforts to obtain consent of the Landlord in a timely manner wherever such
consent is required in the Main Lease.

17.      Sublandlord agrees to use reasonable efforts in a timely manner in
exercising its remedies under the Lease in the event the Landlord fails to
perform its obligations under the Main Lease.

18.      If any provision of this Sublease, or the particular application 
thereof, shall to any extent be held invalid or unenforceable by a court of
competent jurisdiction, the invalidity of such provision shall not be deemed to
affect the validity of any other provision of this Sublease. Such invalid
provisions shall be deemed to be stricken from this Sublease, which shall
otherwise continue in full force and effect in all respects.

19.      This Sublease shall be binding upon and inure to the benefit of the
parties hereto and their respective permitted successors, assigns, heirs and
legal representatives.

20.      This Sublease shall be governed by and construed in accordance with the
laws of the Commonwealth of Massachusetts.

21.      Sublandlord and Subtenant each warrant to the other that it has had no
dealings with any broker or agent in connection with this Sublease except for
Spaulding & Slye who shall be paid by Sublandlord pursuant to a separate written
agreement. Sublandlord and Subtenant covenant to hold harmless and indemnify the
other party from and against any and all costs (including, without limitation,
attorney's fees for defense of an action), expenses or liability for any
compensation, commissions and charges which result from the breach of this
warranty.

22.      Notwithstanding anything to the contrary contained herein, this
Sublease shall not become or be deemed to have become effective until mutually
executed and delivered by Subtenant and Sublandlord and consented to by Landlord
in writing.

23.      ADDITIONAL SPACE.  Sublandlord agrees to sublease to Subtenant 
additional space on the 10th floor of Tower II of the Main Premises under the
terms and conditions contained in Addendum I and Addendum II attached hereto and
made a part hereof.

EXECUTED under seal as of the day and year first above written.

                                       SUBLANDLORD:

                                       NATIONAL MEDICAL CARE, INC.

                                       By:______________________________

                                       Its:_____________________________


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                                              hereunto duly authorized

                                       SUBTENANT:

                                       GEOTEL COMMUNICATIONS CORPORATION

                                       By:______________________________

                                       Its:_____________________________
                                              hereunto duly authorized







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