We have two different style of Cisco VoIP phones in house. The standard IP79xx models, and the lower grade SPA50x models. The phones can ring each other, but when the receivers are picked up, a user on a 79xx phone can't hear or be heard by someone on a SPA50x phone, and vice versa. A user calling from an IP79xx to an IP79xx works just fine, as does calling from a SPA50x to a SPA50x.
I have used the exact same hardware topology and configuration, in other locations and the only differentiating factor in my current design is that I have Cisco 3548P switches instead of the usual Cisco switches.
Does the Dell 3548P switch have a specific signature that it uses for traffic shaping of VoIP traffic on VLAN100 (voice vlan), depending on the packet signature of the VoIP calling device detected?
