Honestly, I'm not a big fan of honesty in dealing with corporations, but in this instance honesty may be your best policy. Tell them you want to be able to come back and be a customer in the future, but due to a change in your earning capability you would not be able to pay an ETF and thus it would prevent you from being a future customer. Offer to send them your disability papers as proof and you have a pretty good shot at getting them to shut you down.
If that fails, your second best shot is to claim to be military being shipped out. By law they can't penalize you. However, by law they can also ask for proof of that (they don't always, but be prepared for that bluff to be called). It's probably best to offer to send it in up front and then say "You didn't get it?!?" a few times and hope they give up.
Third option, take the ETF, don't pay it, take a hit to credit score, never be a T-Mobile customer again. It's simple and effective.