呼叫SIP中继时无法听到振铃信号

I run an Asterisk server with 10 IAX2 extensions (located in different countries). I am able to call make calls between my extensions without any problems. My asterisk server is behind a NAT router. I have the appropriate firewall/port forwarding setup so that my clients can connect to my box from the outside.I run an Asterisk server with 10 IAX2 extension