Question : InterTel phone QoS

I have a question regarding issues I am experiencing with our InterTel phone system.  Here is our environment:  We have a 14 node Intertel phone system connected via IPRC cards over an MPLS network with as many nodes.  In order to give priority to the voice traffic, we set up QoS on our Riverbed Steelhead WAN optimizers (which also exists on all nodes).  Through the Riverbeds, the voice traffic is NOT optimized...it is simply passed through, and all traffic between IPRC cards is given real-time priority.  We do not experience any noticeable jitter, but on an ongoing basis, we do get "dead air" when we 4 digit dial between nodes (e.g. Bob calls Mary, and Mary can hear Bob, but not vice versa).  

Now to my question...does this sound like a QoS issue, or could it be an issue with the actual phone cards?  We are not reaching anywhere near bandwidth capacity on our MPLS network, and I have verified through Riverbed that my Steelhead configuration looks good.  I cannot account for why this happens.

Any thoughts or direction would be much appreciated!

Answer : InterTel phone QoS

This is referred to as one-way voice and it is not a QoS issue. I have no experience with the specific equipment you mentioned, but some general pointers would be :
1) if you are doing any NATing, is it set up correctly to understand voice signaling and do the proper translation on it ?
2) use Ethereal to capture a call between these two endpoints, can you see RTP flowing both ways ?
If not, check again right outside the device which does not seem to 'send' an RTP flow
3) If so, check right outside the device which doesn't hear anything, if it receives the incoming package, this is either a configuration issue or possibly codec mismatch
4) If traffic is lost anywhere in between, check firewalls and as mentioned before NATing