How can I pay my last bill from Australia?

We relocated to Australia on 24 July 2022. Yesterday I received an overdue bill from August 2022, which took 6 weeks to arrive from the USA via snail mail. Why didn't anybody email me?

I managed to pay the bill via Guestpay using my account number. My t-Mobile American cell phone number does not exist anymore, as we now live in Australia and have Australian cell phone number.

Today I received my final bill. again 6 weeks late, but I am unable to pay it as Guestpay insists on verification via my old American cell phone number which I cancelled in August.

T-Mobile cannot be reached by email, and I do not have international calls on my present cell phone.

I can send a check, however, that would take another 6 weeks to arrive in America.

SO: HOW CAN I PAY MY FINAL BILL??? is my question.