Outstanding EIP balance?

My mother has dementia and at some point when her iPhone iOS was updated she lost service on her phone. We called customer support who had us try some things but it wasn’t successful. They said we would have to take it into a store to resolve the issue which had to do with the eSIM. It is very difficult to take her anywhere because of her condition and so that never happened.

She hadn’t been able to use her phone in months because of lack of service so she stopped paying for the service on it. Not out of spite. She just figured she didn’t owe for a phone that didn’t work. After 90 days of non-payment T-Mobile sent her a cancellation letter and a Final Bill which I found amongst her things. She is fine with never using the service again.

She can no longer login to her online account because the phone number was discharged so we can’t see the status of anything. They won’t talk to me on the phone and she has aphasia (can’t speak properly) so we can’t call them together.

When she got the phone I was with her and I asked them to add me as a POA or whatever the designation is to do transactions on her behalf but they screwed that up and apparently never added me because I went into the store to pay and they wouldn’t talk to me despite me having the phone, her ID, all of her passcodes, and her paper bill. I told them I just wanted to pay the bill, not make any changes, but no dice. It was very frustrating to be trying to give money to someone who won’t take it.

I went ahead and made the final payment for her using the Guest option online using her account number.

Finally, my question:

On the Final Bill it says that she owes a $200 balance on her iPhone but they only charged her $100 for equipment on the Final Bill. Why wouldn’t they charge her the entire $200? I don’t want her to have another $100 left hanging over her head somewhere. I am willing to pay whatever is owed but I have no one to talk to about it. Did they just charge that off? It doesn’t say. We want to be able to unlock it at some point and use it on another carrier.

Bonus question:

How do we know if this went to collections? The Final Bill doesn’t make any mention of collections or anything. She received it a month ago.