G729 codec problems

I’ve been experiencing some voice quality problems with a UTStarcom F1000G phone. Voice quality on these phones are very jittery. Having been advised to try the G729 codec instead of G711, I purchased 5 G729 licences.

The Asterisk node show me the licences installed:

ipabx*CLI> show g729 0/0 encoders/decoders of 5 licensed channels are currently in use
Now if I make a single call I can hear nothing. Executing the same command after hangup gives me the following:

ipabx*CLI> show g729 5/0 encoders/decoders of 5 licensed channels are currently in use
What’s going wrong here?

TIA.

Iain.