Outgoing call has 1-way sound and no ringing.

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I paid for voip unlock key and configure it on a sip adapter, which is behind a router/firewall. The sip adapter has 2 phone ports. The first port connects to fongo/freephoneline. The second port connects to other provider using Asterisk PBX. Now I experience the following problems:

- Outgoing call to non-fongo number has outgoing sound only, and has no ringing.
- Outgoing call to fongo number is fine. It has 2 way sound and ringing.
- Incoming calls are fine. 2 way sound. Not matter it is from fongo or non-fongo number.
- Tried voip.freephoneline.ca, voip2.freephoneline.ca. Same result.
- The second port is always fine. Incoming or outgoing calls. Ringing. Any number. All good.

I did not sniffer packets on sip adapter and guess:
- Ringing. Fongo does not pass back SIP 183 ring message properly for non-fongo number.
- No incoming sound on outgoing call to non-fongo number. The router does not recognize SIP message properly and does not a open dynamic port for incoming RTP traffic. But this router works fine with another provider.

Does anyone else have similar problem?

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It is a NAT/Firewall problem. 2 way audio goes through after the phone adapter is put in DMZ.