Dialplan decisionmaking using Caller ID- where is it stored?

Hi,
Which builtin variable/function holds caller ID of an incoming call to a POTS line? If such variable/function does not exist, how to capture caller ID of an incoming call for further decision making in the dialplan?
For example, playing different sound files depending on detected caller ID…

voip-info.org/wiki/index.php … c+callerid