This is correct but not for the reason the T-Mobile sales guy indicated. Its isn't a conflict with the devices so much as UMA or GAN relies upon a feature within the MSC or call server. It is more likely that the T-Mobile network topology is not configured in a way that allows a single MSC or call server to perform UMTS, 2G and UMA concurrently. The radio network be it UMTS,GSM or UMA/GAN is homed to a MSC call server that support your geographical area. UMA to GSM handovers are difficult. Put the soft handovers of UMTS on top of this and it becomes a nightmare to provide a level of service without tons of maintenance to prevent an obscene high number of call drops between radio technologies.