Unusual 1 way audio

Hi all, This is my first post so please be gentle.

We are using asterisk 1.8.22 and have a single remote extension which is behind a NAT router at the user’s home. His extension connects OK but if he dials an internal extension he gets one way audio. This isn’t unusual and I have had this many times before.

What is strange is that if he dials an external number i.e the asterisk routes it out over an IAX trunk, the audio is fine. I would have thought that the call is passing the same route from his handset to Asterisk so it should have the same problem as dialing an internal extension.

Can anyone please enlighten me as to why there is a difference?

Many thanks