Ok - So a very specific question, but I don't seem to be able to find the answer. I just ordered the full price TMobile iPhone 6S Plus. My AT&T contract on my iPhone 5S runs out on the 20th. When the TMobile phone comes can I just pull out the SIM and replace it with the SIM from my 5S and it will work? No activation fee, no calling AT&T? I haven't gone this route before, but have the mobile share plan and want to keep the $25 discount but have an unlocked phone to use here and in the UK. Anyone help?