To call a SIP URI, create a Virtual Extension with a Dial field of e.g.
There is a slight security risk in that the callee will see your public IP address and if he’s evil, may probe it for vulnerabilities.
OTOH, receiving SIP URI calls are a far greater risk, because your firewall must allow your system to be accessed from anywhere. Trust me, let someone else have that headache. Get a trunk from a provider who supports receiving SIP URI calls (Callcentric, VoIP.ms, Anveo, …) and set it up as a DID on your system. I believe that Callcentric can be completely free for this application; the others require a deposit but can be set up so no monthly or per-minute charges are assessed for SIP URI calls.