I know that it is possible to make phonecalls to landlines and probably mobiles as well. You'll need special software to do this though and it will cost you to phone anybody. Different companies have different rates so I'd suggest you search for a good deal.
You could also consider Instant Messenging which offers TEXT, VOICE and VIDEO conversations between multiple users. There are many programs such as MSN Messenger, Yahoo Messenger, PalTalk, ICQ, etc.