Can not hear voicemial prompt audio

I am using Asterisk 1.4.22 as part of trixbox v2.6.2.3.

I am having the intermittent issue of not being able to hear the audio being played when dialling *97 from SIP clients. The usual prompt for the password can not be heard throught the phone handset. But I can always see the audio being played in the asterisk console.

This also happens when I used the paging and intercom service. All the normal stuff of making and receiving calls has not been affected.

Searching through this forums I have come across stuff like making sure echotraining is commented out, which it is by default.

People are getting angry at me in the office, so any help will be appreciated.