I still suggest that it is PoE related but do not know how to prove it.In your situation activity on the phone could be putting additional load on the PoE circuit. The additional load then causes the amps, watts, volts, etc.. to drop below a toleranc...
I am in a similar position to the original poster of this question. Symptoms seem to only occur when a machine with a 10/100/1000 NIC is also in the loop.How would you measure whether or not the PoE is overloaded? Do I really need to get the ampmeter...