Duplicate Tones

Hi All,
I have an issue I cannot work out a solution for. DTMF codes are not being sent consistently. We are using LinkSys SPA942 (Sipura Firmware 6.1.5(a)) Handsets and and asterisk server (trixboxce v2.6.1) which then connects to mynetfone.com.au.

If I set the DTMF tx Method to anything but InBand, our voice mail works, but external phones hear multiple copies of the tones (approx 5). If I set to InBand, external phones hear one tone, but the voicemail system doesn’t work.
If I change the DTMF mode in the sip_additional.conf to dtmfmode=inband, then voicemail works, and external hears multiple tones.

Any thoughts would be great. thanks in advance.


Which codec(s) are being used?