You didn't mention what encapsulation you were using on the WAN link between Net A and Net B, but you would want to use LLQ (Low Latency Queuing) to prioritize the bandwidth required for 3 calls (how much that is depends on the codec used). LLQ documentation can be found at:
You can't tandem switch VoIP calls, only VoFR or VoATM calls.
In that case it seems that the best answer is to add a gatekeeper to the network. Using that, you can have the voice gateways in Net A and Net B be registered in "zones" in the gatekeeper configuration, and it can be configured to assume how much bandwidth is available for the zone. So if all the gateways in each net use the gatekeeper to route the calls, the gatekeeper will not let the call be placed if all the bandwidth is already used up because there are 3 active calls.
I'm not able to access my old voice mail messages all of a sudden. The recording says something like 'the message is currently not available'. This has never happened before in all the years I have been using this system. I have t...