It depends on the radio. Those "simpler" models with fewer keys usually have a call list. This is just a list in which you can select which radio to talk to. THe upper level radios (Keypad), you can have a list, and also input an ID to talk to. Most people probably don't know their ID (some systems are set up really nice can have your unit ID the same as your radio ID: ie: Your Mobile would be 1134 for Unit 134, and your HT would be 2134 or something like that.) but some do, and are listed on the radio. EDACS radios work the same way - you can have a list, or you can punch in an ID also.
Some systems have I-call disabled administrativly - it won't work even if the radios are programmed for I-call. Another thing fun with trunked systems is the page function (makes the target radio beep until you answer it)- no scanners out now (that I know of) pick this up - you'd have to use trunker.