T-mo and 3 are sharing the 3g infrastructure, but it is not something that can happen instantly. As another poster pointed out, it started in nov 08 and is a long and drawn out process. By now many sites will be actively sharing, but still many will not. I can't recall when they say it will be finished by, but I have read that this will continue through the orange merger. Another thing that will continue is orange sharing it's 2g network with 3 as a roming partner