After much head scratching, inconsistent one way audio problems, some routes through the astersisk producing audio some not… I noticed I’d set the port forwarding on the router for the RTP stream as “TCP” not “UDP”. Simple slip of the finger, ticked the wrong box on the router interface.
It caused some really weird problems though!
Any audio path which resulted in the asterisk setting up the RTP stream worked, but anything which relied on the VOIP phone initiating the RTP stream ended up with either no audio, or one way audio. This wasn’t immediately obvious from the pattern of symptoms as reported!
Anyway, I have rectified the error, ticked the right box, and my testing seems to suggest that it’s working now.
I’ll try and fix the next fault a bit quicker, promise!