well, in General, Yes, you need to forward some ports on the router for the server, rather, to asterisk on it. this should be the port that is used on asterisk to receive SIP traffic (typically UDP 5060), and ports for receiving RTP traffic (perhaps this is the range of UDP 10000-20000, but there is more correct to look at the config of asterisk, as set up in fact). if port forwarding actually specify these ports and IP address of the internal host to which they propisyvayutsya (server and asterisk).
how to forward the ports depends on model of router.
also on the server with asterisk for each sip account in the sip.conf, which should be able to connect from the Internet, you will probably need to add the line
there may be other nuances: a firewall, binding the asterisk to the interfaces. but guess it's not interesting, I'm more used to answer the fact.
this is all when talking about the Internet and the port forwarding through NAT.