To answer your question I will first refer you to this excerpt from Vonage's FAQ:
"Can I use multiple phones with the phone adapter?
There are four ways to plug multiple extensions into your ATA. Please
note that currently only phone port 1 is used by our service.
You can purchase a multiple jack extension connector, and then connect
it to phone port 1 of the ATA. This will allow you to have multiple
phones on a single Vonage DigitalVoice line.
Some of our customers use cordless phone systems that come with a base
unit and extra handsets. The base station of the cordless telephone
plugs directly into the ATA. Multiple handsets can be placed anywhere
in the house.
You can purchase wireless phone jacks. The base unit connects to the
ATA and the wireless jacks plug into your household electrical
outlets. A telephone connects to the wireless jack. Please note that
only the phone attached to the base unit will display caller id
information. Also, during testing in our lab, we noticed sporadic
dropped calls on the extension phones and an inability to talk on more
than one phone at a time.
Some Vonage DigitalVoice customers have plugged the ATA directly into
the existing telephone wiring in their homes.
The manufacturer (Cisco) of the adapter does not recommend hooking
anything to the adaptor other than a telephone, but you can seek
advice from an electrician or someone who is familiar with internal
telephone wiring on how to best connect it to your current wiring to
take advantage of multiple extensions. We would advice that you
pre-check the wiring on the jacks to make sure you don't have live
PSTN voltage on the jacks before connecting them to the ATA (this can
be easily done by using a line tester such as those that are available
at Radio Shack). Be advised that any damage resulting from practices
contrary to manufacturers recommendations for the ATA will not be
covered by warranty.
(Vonage has recently switched to a Motorola supplied ATA, but the
information is still valid)
Now, please note that unless you are very familiar with residential
telephone wiring techniques, you may wish to get a telecom specialist
to help you with the last option. In this option, it is necessary to
check, double-check, and triple-check that your in-home wiring is
completely separated from the telephone company's wiring that leads to
the junction box/demarcation point for your home, before attempting to
connect the ATA to your home's wiring. Failure to confirm that the
wiring is disconnected *WILL* destroy your ATA.
I should also take this opportunity to remind you to review the Google
Answers disclaimer, and that ultimately you should seek the advice of
a local electrician or telecom specialist before performing any of the
steps below, since neither myself nor Google Answers are liable should
you damage your equipment.
The steps that the electrician or telecom specialist would need to
take (or you, if you feel that you are familiar enough with telephone
wiring techniques) are:
- unplug all telephones and other devices from all telephone jacks in the home
- open up the junction box (often located outside your home)
- disconnect each of the telephone company's lines from the
terminating blocks, wrapping each wire end with electrical tape for
- ensure that all of the wires for each extension line leading into
the home are secured to the terminating blocks
- use indelible ink on a sheet of PVC sheet to write: "DO NOT
TERMINATE TELCO WIRES. IN-HOME WIRING IN USE FOR PRIVATE NETWORK.",
and attach the sheet securely inside the box in case a telco
technician comes by unannounced one day
- use a multimeter to check for voltage across any of the four
terminals for the in-home wiring (there shouldn't be anything greater
than 1 VDC. If there is, something else is already pumping power into
your in-home wiring)
- close the junction box, and secure it with wire or cable ties
- back in the home, use a standard telephone patch cable to connect a
telephone to an extension wall jack
- using another standard telephone patch cable, connect the ATA to
another telephone wall jack
- pick up the telephone handset that you plugged into the extension,
listen for a ringtone
- if you hear a ringtone, you may proceed to plug your other phones
back into their respective wall jacks
NOTE: The ATA's telephone jack output power was only designed for one
phone. Using multiple handsets simultaneously (ie. several people
picking up handsets for the same call) may degrade the audio signal;
if this happens, hang up some of the extensions.
I hope this helps,
Google Answers Researcher