They ALS0 said my assurian was cancelled, as well.
You switched the sim on the device is why it was cancelled. It's actually spelled out in the package they sent you when you added the insurance.
Word of advice. Even if you switch carriers they all have this stipulation and that is set forth by the underwriter(Insurance company) and not the carrier.
I am currently going though a very similar situation, except I am unable to find that particular stipulation on any documentation. Other companies do not offer Apple Care the way T-Mobile does so my confidence in tidbits' advice is low. If I am mistaken, I am open to any educational opportunities through cited sources. Finding this document would be very helpful.
The most disappointing aspect of this has been the lack of communication on T-Mobile's behalf. I did not receive a 'package' specific to T-Mobile's premium device protection, and was not informed of the cancellation after the fact by any company; so being completely blindsided by this has been incredibly disappointing.
The representatives at the store, over the phone and through chat all assured me that I would be protected with Apple Care, but it took multiple conversations over the period of two days for one representative to actually come to the conclusion the editor of this thread has. So much work, simply to be told it was all my fault; but no worries, they can fix it with refurbished parts for $75 more than Apple would have done. On top of having paid for Tier 4-6 w/ Apple Care since the beginning of this particular line's life, uninterrupted.
On top of it all, I am unable to cancel the Apple Care service from being charged on my bill, and during my last call with the company, lowering tiers was also not an option. Since upgrading my device through JUMP! requires this particular package, I cannot have one without the other.
Terrible customer service, indeed.
One main difference between myself and the editor of this thread is that I still have Assurant protection. That has not been cancelled. Only Apple Care.