Why doesn't t-mobile accept X1 credit cards?

T-mobile doesn't accept my X1 credit card as default payment method. (I even went to a T-mobile store on request of a T-mobile customer service representative and, needless to say, they couldn't get my X1 credit card to work either.) My X1 credit card works everywhere else except for T-mobile. I contacted X1 and was told that lots (all?) of their credit cards get rejected by T-mobile, but T-mobile hadn't not responded to their contact requests. I would like to use my X1 credit card because it provide cell phone insurance.

So, why doesn’t T-mobile accept X1 credit cards, and when will this change?

Gracias.