EXHIBIT 10.1

AMENDMENT 4 TO THE MASTER SERVICES AGREEMENT

BETWEEN

VONAGE NETWORK INC. AND TELECOMMUNICATION SYSTEMS, INC.

WHEREAS Vonage Network Inc. (“Vonage”) and TeleCommunication Systems, Inc.
(“TCS”) are parties to the Master Services Agreement dated June 8, 2005 (the
“Agreement”);

WHEREAS effective this 25th day of June, 2007, the parties wish to modify and
amend the Agreement;

WHEREAS the parties acknowledge that all of the definitions and terms shall have
the same meaning in this Amendment to the Agreement between Vonage and TCS
(“Amendment”) as the definitions and terms in the Agreement;

NOW THEREFORE, for good and valuable consideration, the receipt and sufficiency
of which is acknowledged, the parties agree that the Exhibit A to the Agreement
between Vonage Network Inc. and TeleCommunication Systems, Inc. shall be amended
as follows:

1. Section 2.4.6 identified as “Operator Routing Support Service” within Exhibit
A of the Agreement will include the following additional language:

If a TCS call taker answers a Vonage VoIP 9-1-1 call and the caller disconnects
the call but the line remains active the TCS call taker shall disconnect the
line only in situations where no audio exists on the call. The disconnection of
the line does not relieve TCS of its obligation to (i) contact the appropriate
PSAP and request that emergency services be dispatched to the physical location
on file for the Vonage VoIP 9-1-1 caller, or (ii) obtain a physical location for
the Vonage VoIP caller from Vonage if TCS does not have the location on file and
request that the appropriate PSAP dispatch emergency services to the caller’s
location. To the extent that TCS’ disconnection of a line with no audio is the
cause of a claim by a third party, then Vonage shall indemnify and hold TCS
harmless against and from that claim, provided that (i) TCS promptly provides
written notice of the claim and reasonable cooperation, information and
assistance in connection therewith and (ii) Vonage shall in no event have a duty
to indemnify or hold harmless unless it receives such notice and has the
opportunity to control the response to the situation. Vonage shall regularly
keep TCS apprised of the status of such claim and in no event shall Vonage have
the authority to admit liability on TCS’ behalf, compel TCS to admit liability,
or agree to a settlement that would require TCS itself to make a payment.

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2. Amendments. This Amendment may not be modified, except via a writing executed
by both Parties.

3. Entire Agreement. This Amendment (a) constitutes the entire understanding of
the Parties with respect to the matters addressed in it and (b) supersedes any
and all prior representations, agreements, or understandings, whether oral or
written, relating to those matters.

4. This Amendment may be signed in any number of counterparts with the same
effect as if the signatures thereto and hereto were upon the same instrument.
Electronically delivered and scanned executed documents will be considered
originals for purposes of confirming an enforceable agreement.

IN WITNESS WHEREOF, the Parties have executed this Amendment as of the date
first written above.

 

VONAGE NETWORK Inc.     TELECOMMUNICATION SYSTEMS, Inc. By:  

/s/ Timothy Smith

    By:  

/s/ Richard A. Young

Print Name:   Timothy Smith     Print Name:   Richard A. Young Title:  
President, Vonage Network     Title:   Executive Vice President & COO