What am I supposed to do?

I switched from T-mobile about 6 months ago, and moved my number to another carrier.

About a month ago, I started getting calls from T-mobile at least once a day. If I answered, there was silence and the call would end after about 5 seconds. If I didn't answer, I would get a voicemail from T-mobile. It was a recorded message that warned that I owed money and my service would be suspended. It gave button options to pay or change languages.

I called the number and it was the T-mobile bot asking if I was calling about the account associated with my number. The only option offered was to pay the bill.

I thought this must be a scam, but a couple of days ago, I got a bill from T-mobile for the same amount mentioned on the recording. I called the number on the bill, and it went to the same bot. The only option is to pay.

I think this is their mistake, but I don't know how to contact them. Many companies have put up a firewall to keep customers from reaching them, except to pay. How many unpaid hours am I supposed to spend to correct this giant company's mistake? Do I ignore it and let it go to collections? WtF?