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CALL WAITING & YOUR MODEM (Updated 1-Jan-03)
Many of today's modems can recover from significant line noise hits - including the call waiting tone that you may get if you have the call waiting feature on your line. The V.92 modem standard allows the modem user to decide how to handle call-waiting: ignore, disconnect, or, when connecting to a V.92 ISP, to place the Internet connection on hold and take the call. See: V.92 Modem on Hold.
If you have a V.90 or older modem, your options are to disable call waiting so your line is busy, use a third-party Internet call waiting service, or experiment with the S10=# init string to try and get your modem to disconnect on call waiting. In some cases, you may be able to upgrade your V.90 modem to V.92 with a driver upgrade, or, consider purchasing a new V.92 modem.
If you have a Lucent-based Win Modem (aka LT Win Modem, Apollo or Mars chipset, 164x), you may be able to use Modemsite's 8.02 "Special" V.92 driver and get disconnected on call waiting. See this page.
V.90 Call Waiting Solutions:
If you have call waiting, in most areas, you are able to disable it prior to placing a call by dialing a code (usually *70) before dialing your ISP's number. If a person tries to call you while you are on the net, they will get a busy signal.
If you don't disable call waiting and your modem doesn't disconnect on the call waiting tone, the caller will hear your phone ringing and ringing and ringing.
You might be able to get your modem to disconnect on call waiting tone by lowering the modems disconnect on loss of carrier register. Consult your modem documentation for the register and values. Most modems use S10 for the disconnect on loss of carrier with a value of 7(tenth's of a second); you could try a lower value (down to 1) by adding s10=# in extra settings. Similarly, if you're getting bumped when a call comes in and you want the modem to stay connected, you would increase the value.
If you absolutely must know when a call-waiting call comes in and don't have V.92 support, and the s10= setting doesn't help, you can always leave the modem speaker ON (m2 in extra settings) which will allow you to hear the modem throughout the connection; if a call-waiting call comes, you will be able to hear the distinct sound of the call-waiting tone and interruption of the modem connection.
Rockwell/Conexant HCF modems: documentation indicates the modem will disconnect on call-waiting tone if you set s10 equal to or greater than 16. (s10=16). The modem must also be set for operation with the US country code. This feature does not work as advertised for most people --- the modem doesn't disconnect even with the s10 setting; although I now do have 1 report that it works as advertised. [See this page for where to put extra settings.]
Call-waiting compatible modems: Before V.92 was introduced, some vendors developed non-standard V.90 call-waiting modems. Actiontec and Zoom made V.90 "call-waiting" modems using a Lucent Venus chipset - these give you a short amount of time to answer the incoming call and optionally switch back to data-mode or disconnect and continue the voice call.
A number of telephone companies, and even third-parties have, or are working on features that eliminate the busy or no answer situation. These solutions may include notification on your screen of a call waiting, and/or forwarding the call to voicemail. One of the solutions available now is Internet Call Manager.