Two types of caller id (the specification, not the program) exist.

Original caller id is now known as "Type 1".  This type of caller id works when the phone line is not in use.  In other words, no one (and no thing) is using your phone line, and someone calls you.  Devices which support Type 1 caller id must monitor the phone line when they are not using the phone line.

Type 2 caller id works when the phone line is already in use on a call.  This is sometimes referred to as "call waiting id", which is often what the phone company calls the service.  Virtually all devices that support Type 2 caller id only work when the device is using the phone line.  In other words, a phone with Type 2 caller id support will show the information for an incoming call if the phone is already connected to another caller.  A separate, identical phone connected to the same phone line will not report the Type 2 caller information if it is on hook (i.e., not picked up).  Neither phone will show Type 2 caller id if the phone line is not use and a call is received.  Most phones with Type 2 caller id support also support Type 1 caller id.  The same cannot be said for modems.

Type 1 caller id requires extra circuitry to monitor the line for caller information before the device accepts the call.  Type 2 caller id does not require this circuitry, so it is easier and less expensive to support.  Consequently, many new modems support Type 2 caller id but not Type 1 caller id.  This limitation means these modems are physically incapable of reporting caller information except when they are in use connected to an internet service provider.

Ascendis Caller ID only supports Type 1 caller id.  This is a direct result of the fact that no modems (at least none that we are aware of) provide TAPI support for Type 2 caller id.  This may be a limitation of TAPI, or of the modem manufacturers unwillingness to implement it.  Even if  Ascendis Caller ID were to talk to the modem directly, it would probably still be unable to access the Type 2 caller information since Type 2 only works when the modem is connected to another modem, at which time Ascendis Caller ID would not be able to talk to the modem since it would be in use!

Many modems now advertised as supporting "caller id" in fact only support Type 2 caller id.  This makes buying a modem that works with Ascendis Caller ID more difficult.  Ascendis Software LLC includes a short list of compatible modems on their web site at

http://ascendis.com/callerid/modems.php

If you're buying a modem that's not on the list, be sure to check for Type 1 caller id compatibility, or buy it under the condition that you can return it if it does not support Type 1 caller id.