Are there any issues with taking a T-Mobile iPhone 5 to AT&T's network? Currently have the no-contract option with T-Mobile, but circumstances might see the phone switched to AT&T for billing/accounting purposes. But, will the phone work on AT&T's network: just fine, with some loss of speed, or not at all? Will HD voice work with AT&T. It might not per some pundits, but it is hard to find an answer with all the white noise in a search. Also, anything else relevant to this switch?