"Alien" audio when reaching voicemail

We have an issue with our system when leaving voicemail message on people mobile phone voicemail messages. The issue only seems to occur when calling a UK Vodafone number.

Outbound Calls
Calls Only to UK Vodafone

We get a clear line with no issues, hear the vodafone announcement, then the tone. Immediately after the tone we hear a garbled noise ‘Alien’ or ‘Darlek’ kind of thing.

Has anyone seen the same and resolved it?



I am wondering if this issue might have something to do with the DTMF tones? For example it only seems to get corrupt audion once the vodafone ‘tone’ is played.

Any thoughts?