What happens if a person does not accept the Transfer of Billing Responsibility?

If you’re the account holder and want someone off your account, and they refuse to accept a TOBR, then you can’t make them accept one.  However, the phone number is yours to do with what you want.  You have the power to simply cancel the line.  

If she doesn't take over the line, then the account owner can choose to cancel it. You cannot force her to take it with her.

Be aware everything on the line until she takes it over is the current account owner's responsibility. That includes the remaining balance if the phone is on installments.

What happens if you have the phone unlocked so you can go to another carrier without accepting the transfer??

Again, the account holder is financially responsible.  If your phone is unlocked, you can port it to another carrier, but be aware that you’ll need the account number and a port PIN to actually port the number.  Also, the number needs to be active when ported, so you can’t do it if the number is cancelled.

I have a separate active number associated with the phone … multiple eSIMs with separate account & phone number so I’m good as far as not being on the hook for the cost of the phone as long as I’m not the responsible party