I recently upgraded to an iPhone 6 on contract with AT&T and then switched to T-Mobile. I picked up the cheapest pre-paid phone I could from Target, a $15 AT&T GoPhone flip phone. Traded that in (got $1 trade in) and had to buy the cheapest T-Mobile phone I could ($30) and pay $10 for the SIM card. No issues trading in the cheapie flip phone and AT&T actually unlocked my iPhone 6 before I even paid the ETF. The unlock request is done through their website.
floats, did you fill out the unlock request form on ATT immediately after switching to Tmo and was approved for unlocking? I have been wanting to switch to Tmo and do what you did, but based on what I gathered so far, one has to wait for the ETF to show up on final bill and then pays it off, before ATT would grant the unlock request. This means I would have to fall back to some old phone (or the cheapest TMo phone bought) for a good 2-3 weeks, depending on when i switch during the monthly billing cycle I guess, before I get to use my then_unlocked_iPhone 6 again. This has been the main reason I have not taken the step to switch. Has anyone successfully had ATT unlock their phones after switching but before paying off ETF?