home internet line getting added without requesting for it

When we added new accounts to my line 2 years ago, T-Mobile added home internet line for 30$. When asked they said it was an accident and they will remove it. I trusted them they would remove it. Since it was on autopay I did not check. Now that I identified that home internet line is still there for 2 years I called them and asked about it. They are now saying it's too long and we can only refund for 2 months. And more frustrating part is they sent me a email to return the device which we never got for a service which we never asked for and never used.

the rep was saying it was my responsibility to check the bill and notify them of any discrepancies.

as a customer am I responsible to check their work. How can this even be ethical. Looks like I am not the only one this is happening with. Does anyone on T-Mobile management even know this. Are they doing it in on purpose to meet their numbers for commission?