CallerID implementation varies from modem manufacturers.(thinking that you are using your modem to have teh caller id).
Because of that, in order to activate the callerID you need to inform the modem with a command. You will need to find out how you modem passes the information through. You might be able to get the information using hayes commands.Or maybe it could be something like;
AT#CID=1 (or AT+VCID=1)
You might want to test your modem for that.
How to Test a MODEM for Caller ID Support
But there is one other issue that you should take into consideration too:
You have to double check with your carrier or provider for the caller ID support. A basic POTS line won't include this information unless the carrier has done some extra work to add it. For that support you need to pay a fee.
Also another helpful link with some code about this. check here