we had this problem with our setup. Turned out that we needed rules in a firewall for not only the MCD IPs but also the RTC and E2T IPs, so check those as well. Also, you didn't indicate if you are dealing with IP phones or analog. If IP, then alot of the comments above make sense; the two devices talk to eachother, but if analog to IP, or analog to analog, then the RTC/E2T get involved.