<PAGE>   1

                                                                  Exhibit 10.49

                            THIRD AMENDMENT TO LEASE



        THIS THIRD AMENDMENT TO LEASE (the "Third Amendment") is made and
entered into JANUARY 22, 1996 by and between RIGGS NATIONAL BANK OF WASHINGTON,
D.C. As TRUSTEE OF THE MULTI-EMPLOYER PROPERTY TRUST, A TRUST FORMED UNDER 12
C.F.R. [Section]9.18 ("Landlord"), and TELCO SYSTEMS, INC., A DELAWARE 
CORPORATION ("Tenant").

                                    RECITALS:

        A. Landlord's predecessor-in-interest, Pactel Properties, a California
Corporation, and Tenant entered into that certain Standard Triple Net Industrial
Lease dated May 3, 1990 (the "Original Lease"), covering certain premises
consisting of approximately 62,261 rentable square feet (the "Original
Premises") located in Building 10 in the project commonly known as Northport
Business Park ("Project") and more particularly known as 4305 Cushing Parkway,
Fremont, California (the "Original Building").

        B. Landlord and Tenant entered into that certain First Amendment To
Lease, dated March 31, 1995 (the "First Amendment"), covering certain Premises
consisting of approximately 85,000 rentable square feet (the "Premises") located
in a building to be constructed of approximately 115,000 rentable square feet
(the "Building"), and more particularly described in the First Amendment.

        C. Landlord and Tenant entered into that certain Second Amendment to
Lease, dated May 8, 1995 (the "Second Amendment"), expanding the rentable square
feet to include the entire building, and more particularly described in the
Second Amendment.

        D. Landlord and Tenant wish to agree on the rentable square feet in the
building, adjust the rent, adjust the Tenant Improvement Allowance, modify the
overage payment terms, and make other related changes as described below.

        NOW, THEREFORE, for good and adequate consideration, receipt of which is
hereby acknowledged, Landlord and Tenant hereby amend the Lease and agree as
follows:

        1. DEFINED TERMS. All capitalized terms used in this Third Amendment
shall have the meanings given in the Original Lease, unless otherwise defined in
the First Amendment, the Second Amendment, or herein. For purposes of this Third
Amendment, the term "Lease" shall be defined herein to include the Original
Lease, the First Amendment, the Second Amendment and this Third Amendment.

        2. SCHEDULE A - BASIC LEASE INFORMATION. SCHEDULE A - BASIC LEASE
INFORMATION, which is attached to the Second Amendment shall be modified as
follows:

              a.    Paragraph 1.4; 2.1 Premises shall be deleted and replaced
                    with the following: 

                    Premises shall be 118,348 rentable square feet.

              b.    Paragraph 4.1 Base Monthly Rent shall be deleted and
                    replaced with the following: Pursuant to the terms of 
Paragraph 2.1 of Exhibit D. Initial Improvements Of Premises.



<PAGE>   2

Tenant hereby exercises its option to have Landlord amortize an additional $3.00
per square foot over the Lease Term as additional Rent at an interest rate of
10%, annually. The Monthly Base Rent shall be increased by $4,653 per month
throughout the Lease Term. and such increase (along with the increase due to the
revised square footage noted above) is reflected in the following Monthly Base
Rent schedule:

                            Month of Term     Monthly Base Rent
                            -------------     -----------------

                            1 through 36      $81,579.00
                            37 through 72     $90,810.00
                            73 through 108    $101,154.00
                            109 through 120   $112,728.00




<PAGE>   3


           c.     Paragraph 20 Security Deposit shall be deleted and replaced
                  with:

                  Upon substantial completion of Landlord's Work, Tenant shall
                  deposit with Landlord an amount such that the total Security
                  Deposit shall increase to $108,075.00.

        3. EXHIBIT D. INITIAL IMPROVEMENTS OF PREMISES. The first sentence of
Paragraph 2.1 of EXHIBIT D. INITIAL IMPROVEMENTS OF PREMISES, as amended by
PARAGRAPH 4 of the Second Amendment, is hereby deleted and replaced with the
following: Pursuant to this EXHIBIT D. INITIAL IMPROVEMENTS OF PREMISES and the
terms of this Third Amendment, Tenant's hereby exercises its option to have
Landlord amortize an additional $3.00 per square foot over the Lease Term as
additional Rent at an interest rate of 10%, annually. Therefore, as its
contribution to the cost of Landlord's Work, Landlord shall provide to Tenant a
tenant improvement allowance of up to a maximum of $2,722,004 (based upon $23.00
per rentable square foot as specified in the Lease) ("Tenant Improvement
Allowance").

        4. EXHIBIT D. INITIAL IMPROVEMENTS OF PREMISES. The last two sentences
of Paragraph 2.1 of EXHIBIT D. INITIAL IMPROVEMENTS OF PREMISES shall be deleted
and replaced with the following: Provided an acceptable financial security
vehicle is agreed to between Landlord and Tenant, in the event the tenant
improvement costs exceed the Tenant Improvement Allowance, Tenant shall have the
option to either pay for the difference between the tenant improvement costs and
the Tenant Improvement Allowance (the "Excess Cost") upon completion, or have
Landlord amortize the Excess Cost (not to exceed a total of $6.00 per square
foot) over the Lease Term as additional Rent at an interest rate of 10%,
annually. All amounts over $29.00 per square foot shall be paid by Tenant as
provided in section 7.1 of this Exhibit D.

        5. AUTHORITY. This Third Amendment has been duly authorized and executed
on behalf of Tenant and Landlord and is valid, binding and enforceable on both
parties in accordance with its terms.

        6. ORIGINAL LEASE. Except as amended hereby and in the First Amendment
or the Second Amendment, the terms and conditions of the Original Lease shall
remain in full force and effect in accordance with its terms.



LANDLORD:

RIGGS NATIONAL BANK OF WASHINGTON, D.C.
AS TRUSTEE OF THE MULTI-EMPLOYER PROPERTY TRUST,
A TRUST FORMED UNDER 12 C.F.R. [SECTION]9.18

By: /s/ Maria Fleming
    -----------------------------
    Maria Fleming
    ITS: Senior Trust Officer



<PAGE>   4
TENANT:

TELCO SYSTEMS, INC.,
A DELAWARE CORPORATION


By: /s/ John C. Kempf
    -----------------------------------
    John C. Kempf
    Vice President - Controller
      Fremont Operations







                                        2




<PAGE>   5

TENANT:

TELCO SYSTEMS, INC.,
A DELAWARE CORPORATION


By: /s/ John C. Kempf
    -----------------------------------
    John C. Kempf
    Vice President - Controller
      Fremont Operations






                                       2