Speaking from experience...
....if T-mobile is anything like Sprint/Nextel this could be your answer....
With Sprint/Nextel you can have more than one user name associated with a cell phone number. Your X may not even be getting into your phone records with YOUR user name and password. She could have created an account of her own with your phone number (all she'd need is your account # and more recently your SIM card number...up until about * year ago you didn't even need the SIM card number to set up an online account...just the account #). So, there could be another user name associated with your phone number and you wouldn't even know it. So, changing your password on the user name that YOU know isn't going to do anything to prevent her from logging into the other account that she could have started.
In my case, when I called Sprint/Nextel they were useless. Complete idiots. They couldn't find out how to search for another account associated with my phone number. Same thing with the "court order" crap....and unless a "crime" is committed the police are also useless. Just recently I found out the other USER name and got it deleted. I also changed my phone carrier and phone number.
Have you thought about a keylogger to see how she is getting in and what else she is up to? Of course that's not legal...;)
My s***estion is to get a new phone company and new phone number and make sure she never gets her little paws on it!!
Good luck!
spytector and most keyloggers, and RATs
all show up on mcaffee as PUPs, potentially unwanted programs. even the newest version of Abel shows up as a pup on mcaffee...:D