Intermittent one-way audio outbound on B400P

I've got a B400P, with two of the four spans in use, connected to BT ISDN2e lines. Intermittently (approx 1/3 of the calls) an outbound call will happen with one way audio, where the remote party hears silence (i..e the audio being sent out through the ISDN line is disappearing)

I'm seeing it on both of the connected spans, and it seemed to start all of a sudden, with no changes to the software configuration.

Using dahdi_monitor I can see both parts of the audio (that coming in from the ISDN line and what is supposed to be going out to it), and I've verified that it's not a SIP issue etc.

The carrier has run their fault diagnostics which claim the lines are working fine - I don't have a spare card at the moment (though I'm going to get hold of one) to try swapping it out, so I was wondering if anybody has seen anything like this before or has any suggestions of further debugging I can do?

HI, i will help you , please add my skype : dario.hu2

Hello,
I seem to have the exact same problem with a B200P card on an elastix 2.3 pbx configuration.
Does anyone know the cause of this, or what should I do to find it?

Hello

Please give us more details, When make oubgoing calls remote side can not hear anything,right? Add this skype dario.hu2, He will check it for you .

That is correct. Already added. thank you for the reply