I investigated the differences in coverage, and AT&T's 3G coverage is slightly better than T-Mobile's. Interestingly, T-Mobile's Edge (2G) coverage was slightly better than AT&T.
Having done this, There are other carriers (MVNOs) that will let you "bring your own phone", so your options are not limited to T-Mobile. Understand that these MVNO carriers use the infrastructure of AT&T and T-Mobile, so your coverage will not get any better than it would be with either of those carriers
I have some observations and suggestions for those that may want to consider changing carriers :
- If you are planning to use unlocked phones, make sure you can get them unlocked. AT&T policy is that they will unlock iPhones that are out of contract with no outstanding charges (including ETF fees)
- Carefully calculate your ETF liability. Payback is much longer if you have a substantial amount of contract remaining.
- As contracts on your phones expire, get them unlocked, then test with a non-AT&T SIM to confirm the unlock
- If you wish to keep your existing numbers, initiate the change process at the destination carrier. If you cancel service for your existing phone lines, the numbers will go back into the pool of available numbers
- As for timing, I would switch less than a week before your billing date. This will minimize the time you will have to wait to close out your old account. If you will need to pay ETF fees, the unlock process may be delayed until the final bill was paid. I mistakenly switched a few days AFTER my billing date, and had the potential of having that third phone unusable for 3 weeks. Fortunately, AT&T was cooperative and performed the unlock once I called them to explain the situation. I did have to call them twice, but everything was very cordial.
So far so good. The only thing I miss so far is being able to forward emails to SMS. Setting this up in GMail involves typing in a confirmation code to verify that the destination address is valid. Unfortunately, T-Mobile's gateway cuts off the message after 160 characters and the confirmation code does not make it through.