When I'm connected to my Home router I see the UMA icon at the top and a signal strength indicator to the right of it. At those times I understand that the phone uses the router to somehow (I still don't get how) connect to Tmob cell towers for calls, thus freeing up their cell connections. It's not VOIP which only uses the internet. But the minutes still come off of my numbers because I don't pay the $10/month for the tmobile hotspot at home plan. When out of range I see EDGE icon on the phone.
BB 8900 Tmobile, Moto Droid Verizon