Extension disconnects whilst ringing outbound

Hi all
I have an Asterisk server running 13.14.0 on Ubuntu on a VM droplet on Vultr and using chan_sip.

A new phenomenon is that when using an extension to dial an external number (over IAX to another * box then to our carrier), the extension plays the ringing tone and the call is made. Whilst still ringing at the external number, the extension just hangs up. The call continues to ring and can be picked up at the recipient but, obviously, there is nobody there so the call gives up.

I’m not sure how to diagnose the fault any more. I have carried out a sip trace but it doesn’t seem to show anything (to me).

Has anybody come across anything like this and any ideas what to do or how to diagnose further?
Many thanks
Mike