Recently we made some changes to our media resource configuration in CallManager 6.x. Specifically, we stopped using DSPs on a 6608 blade in a Catalyst 6500 and started using DSPs in a couple of 2811 routers. Immediately after this change, a couple of users reported that they could no longer make outbound PSTN calls that transit a particular H.323 gateway. The weirdest part was that if they rebooted their 7940 phone, they could make a single call, but all subsequent calls would fail with a fast busy until they rebooted again. Calls through other gateways worked fine.
A ISDN Q.931 debug showed that the second outbound call was setting up as "unrestricted digital" instead of "speech".
I went 'round with TAC about this for a few emails and they were getting ready to blame the telco (?!) when I realized that the thing these two users have in common is VT Advantage (a Cisco desktop video-conferencing application). I asked them to turn off VT Advantage, and presto, all calls now worked.
Upgrading to the latest VT Advantage version solved the problem completely.
Why a change in media resources would affect outbound calling through a single H.323 gateway on users with an old version of VT Advantage is beyond me, but that's my story and I'm sticking to it.