Basically I have this iPhone 6 Plus that's been off for a year because it's been under my moms named and she has passed?

I have tried to go into Tmobile and tried to pay the bill off but the guy said the collection agency has my bill. Can someone tell me what I need to do to get this phone paid off?

In that case it is not your bill, it is your mom's. Call the collection agency, tell them she died but the phone was yours, see what they tell you.