>I need to know if it is posible to get the caller phone number from a modem that is connected to a telephone line with the caller id service (this is an special pay service in my country). I want to use it to identificate a delivery bussines client on my program.

It depends on the modem & the caller id system used by the exchange. The following sites may be of help :

http://www.ainslie.org.uk/callerid/cli_faq.htm
http://www.modemhelp.org/

The AT command used by several modems is

AT#CID=1 (formatted)
or
AT#CID=2 (unformatted)

Note that the caller id information is sent in the incoming ringing tones, so you have to allow at least one ring of the phone.