We had the exact same problem. CallManager was setup right, the router was config'd right, etc.. Calls would come in, but when you tried to call out you got dead air. We verified the port was going off hook and the call was progressing (by doing a trace).
Also verfied the lines were ground start and working properly by calling out on each line with a but-set. I finally remembered that ground start lines are sensistive to polarity (doesn't matter as much with Loop Start). I had them flip the pairs and the calls went through.
