Who is the cellular carrier on your bill?
Realize that you are the person responsible for the phone. If she's not paying you - call the cell carrier & ask them to "suspend" the line. Make sure your account has a passcode so she can not call in & pretend to be you & have the phone re-activated.
Tell her she needs to pay the past due balance in full before you will turn the phone back on. Do NOT allow her to transfer the phone number out of your account & onto someone else's account! Your only leverage is the fact she likes to yack on the phone, and she'll pay to get that number turned back on.
If you go to court, be prepared to PROVE that any overages are due to her use. Detailed billing showing calls she made & her actual minutes of use will help your case. You usually must pay the carrier to provide a detailed bill from past months.
Finally, I hope you realize why your 'friend' couldn't get a line under their own name. The carrier ran their credit & knew they were probably not going to pay.
Next time, give your 'friend' $20 and tell them to buy a prepaid phone. It will be cheaper & less headaches for you.
Good luck.
-Andy