You are right, we hunt down to the second PRI if 23 channels are being used in the first PRI.
In the scenario where the first PRI is up, but not operational, we do try to setup a call. If call goes into a connect state and then drops, you are out of luck. If the PRI setup message gets an immediate disconnect from the other end (which means we did not change MGCP state with a MDCX, or if it is an H323 GW, we did not go to proceed), then, we will hunt to the next available channel.
Hope this helps.
Regards,