You can with SIP. The only thing is that you need a SIP server to hold your number. As far as i know moder telephony is internet telephony. SIP servers establish connection between 2 clients. SIP servers hold a database with phone numbers, when you call someone you ask which is the current ip for a given number. SIP servers are the ones that mantain the link between a phone number and an ip (beyond other link related stuff).
Without a global server that holds a number phone for your current ip you wont be able to get calls or even make it.
But you can create a SIP server for your friends probably. That's how the FSF has it setup.