We had a similar issue when we installed our mitel system (random calls dropping or not able to here other party internally or externally). We have two VLANS. 1 the default for PC's and 2 for the IP Phones. The 3300 was originally set up for some unknown reason in VLAN1 (all the phones in VLAN2) and both the 3300 and phones were (again for some unknown reason) setup to use our D/G (being our Internet router) to route all the traffic internally. So the path of traffic from the phone to the 3300 was phone > D/G > Layer 3 Core Switch > 3300 and vice versa for back again. Once mistake spotted, 3300 moved to VLAN2 so path for traffic from phone to 3300 is now phone > 3300 and of course vice versa for back again.