We have had IP phones deployed through a network of 200 ICP's (5-ICP's) for about 2 years. We were using HP 2620 POE switches with VLAN 180. Everything has been running fine for about two years. The customer replaced all swicthes with Cisco 3750 POE 4 months ago. 2 sites now have constant issues where an extension dials another extension (local ICP) the set rings but they cannot be connected. When the user picks up the handset it keeps ringing. This usually last 10-12 seconds. We have pulled numerous logs with Mitel support, pulled wireshark cpatures, I even hired a Cisco sub contractor to help us trouble shoot. I am pretty sure it is the Cisco switches, I just cannot prove it. The logs indicate IP network congestion on the IP trunks everytime the user has an issue, however the user is not using an IP trunk, these are local ext calls. The customer has a feature call Port security on the switches. Does anyone know what that is and if that may have any effect on the Mitel phones? The phones are all 5212 and 5224.