If your gateway is MGCP and your service parameter to advertise G.729 annex-b, which is the default, you will not be able to hear audio on 12.4(24)T.
Becoming kind of a hot bug, thought I would let you know.
Calls on an MGCP gateway negotiating g729br8 may fail to have audio in one or both directions.
This occurs on MGCP gateways with the fix for CSCsu66759 when g729br8 is being negotiated.
Any of the following will be sufficient to get around this issue:
1. Configure the gateway for static payload type using the following commands on the gateway:
mgcp behavior g729-variants static-pt
mgcp behavior dynamically-change-codec-pt disable
2. Disable g729br8 from being negotiated for this call. If CUCM is involved, this is done with the service parameter 'Strip G.729 Annex B (Silence Suppression) from Capabilities'
3. Downgrade to IOS code on the gateway which does not contain the fix for CSCsu66759, which would be 12.4(22)T and below.