Is it busy going to or from the CUCM? You didn't mention what the other building is running.
My guess is you don't have the SIP trunk on the CUCM enabled to be active on all nodes if it's busy going to the CUCM. This means calls can only go to the first CUCM listed in the CUCM group specified in the device pool. If a SIP call is sent to another server in the cluster, they ignore it because the first server in the list is active and should be processing all calls. So if the other end is rotating calls between two CUCMs, the active one will accept the call and the other will ignore it. By making it active on all nodes, any CUCM in the cluster will accept the call.
Also note, that having it active on all nodes already could also cause this if the busy signals are happening when calling the other building. When active on all nodes, any CUCM in the cluster can send the call out. If the other end doesn't have all the CUCMs in the cluster configured, it may ignore the call because it doesn't recognize the CUCM IP as being part of that SIP trunk.