Find me Follow me only works with Fax Detection turned on

We had our PBX system installed about 18 months ago. The vendor did most of the setup, but I learned a little bit along the way and have been doing my best to maintain the system since. (I am an engineer, but have no background in PBX, so please forgive my lack of experience.)

For a couple of employees, I have Find Me Follow Me setup to ring simultaneously at both the users phone and their cell phone. This was working fine for a while, but at some point quit working. If the user tried to answer the call on their cell, there was no audio in either direction.

The fix was to turn on “Fax Detection” for inbound routes. Any idea why this solves the problem? Any side effects I need to be concerned with? We are running PBXact

This is a work around, not a fix. Your router is mis-handling the SIP media (the audio part of the call). Incoming SIP media packets arrive at the router from the WAN, but the router doesn’t know what to do with them so they are dropped. Once the PBX starts sending outgoing media to the provider, the router realizes that the incoming media is intended for the PBX and finally starts allowing it in. In cases where the call is forwarded off the system via fmfm, there are no outgoing media packets from the PBX to establish a NAT path thru the router so all audio fails. Answering the channel by means of the fax detection causes media packets to be sent outbound from the pbx thereby fixing things again. The answer is often used in conjunction with setting the chan_sip parameter, progressinband which also forces outbound media thru the router.

The actual fix is to config the router to port forward the entire Asterisk RTP port range (default UDP 10k-20k) inbound to the PBX.

Thank you. I will look into the port forwarding settings on my router.

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