SIP and carriers · advanced · 12 min full read
NAT and PAT are acceptable. Double NAT is not. UDP bindings that idle out cause one-way audio. The requirements use a 30 second UDP idle timeout as the planning number, and they tell you to send keepalives when the binding is shorter.
On a VPN, translate RFC 1918 space to a public address the far side can route. Do not exclude Five9 domains from a split tunnel and then wonder why signaling dies. Do not restrict SIP or RTP between sites.