Unless your phone system is just plugged straight into the internet [ie, has a public IP address], it won't have one. You would need to NAT some ports to your phone system on your edge device [eg firewall], but you want to think very, very carefully before allowing the whole internet in on port 5060. And by think carefully, I mean you almost certainly don't want to do this - you either want to restrict it to a known whitelist of public IPs that users are allowed from, or if you don't know, then you need something like an MBG in the middle to protect the phone system.
