Understanding billing credits

So I had T-mobile internet service for a year, then decided to change plans. Payments were set up as auto pay at my local credit union. I close the account, return mail the equipment, and then for two months I’ve received a “account credit” notices in the mail showing I have a $41.51 credit. I finally call today (after visiting my local store and getting shrugs as to what to do), and after I speak with an agent, she asks me for the complete debit card # as for a means of readjustment I’m assuming. The second issue is that shortly after I’d changed providers, I also changed my banking. The agent said I needed to have the complete (debit card) number in order to complete my request.

I called my prior banking institution, and they were kind enough to give me the rest of my debit card information (even though they’ve blocked me from seeing any of my prior 7 year financial transactions), I returned called this hotline, this time speaking to another agent, this time she says she doesn’t need the debit card information, but she does need me to go to my local store and obtain a “key code” authorization to prove I’m who I say I am (??)….she say’s she’ll call me when I’m at the store to help communicate with the staff there…..I go there, but she either didn’t call, or I missed the call, so I redialed and spoke to the 3rd agent. He confirms after speaking to the in-store rep that I’m who I say I am, then he begins to ask why I’m getting a credit on my account(???), then after a several minute discussion about why this $41.51 dollars really isn’t a refund that can be “refunded” to me via bank transfer…..

Ok, questions:

#1.  the statement I received from Tmobile for 2 months (after my service had ended) clearly indicates that I’m no longer a customer, and at the same time it also states that this “$$ credit” will be reflected on my next billing statement(???) How does this make any sense?

#2. If the company clearly understands I’m no longer a customer, instead of sending me a billing statement showing me the credit amount, why don’t they just send me a check for that amount?

#3. what useful purpose (in my perspective) does this notice serve?….and what am I to do with it?

…..deeep sigh…..ok, it appears I’ve wasted  a lot of time today (drove across town, more gas, time and aggravation), all in (futile at this point it appears) an effort to straighten out a credit that was due me. I’m pretty sick of the whole matter right now to tell the truth, but I’m hoping I can get SOME kind of resolve from this email…..

John C.