SIP and RTP source NAT
In the source NAT scenario shown below, a SIP phone connects to the Internet through a FortiGate with and IP address configured using PPPoE. The SIP ALG translates all private IPs in the SIP contact header into public IPs.
You need to configure an internal to external SIP security policy with NAT selected, and include a VoIP profile with SIP enabled.
SIP source NAT