This works perfectly with the exception of not being able to send DTMF
tones over the line - dialling is no problem. Therefore I'm unable to
call most business lines which answer with "For xxxx please press y".
This also rules out voicemail.

I've changed the analogue handset to an alternative which makes no
difference. The ATA186 is running Version: v3.1.0 atasip (Build
040211A).

Any ideas, please?