we went through similar with my 14 yr old stepdaughter. she got the phone for helping with firewood. the deal was as long as she helped, she got to use it. she wasnt allowed to use during the week so she could concentrate on hw. weekends she could use it all she wanted, with 2 major rules. she could not call or text boys and we were allowed to check it whenever.
one day she decided she didnt want to do firewood anymore. but she had babysitting money to pay for it so we let her, but she didnt understand why we were still enforcing rules. she broke the no using the phone after 9 a few times and finally we took it away indefinatly. in august when we were going on vacation to see my family. she wanted to stay at her dads. we gave her the phone to stay in touch with us. upon review of the phone bill and the phone itself we found out she was calling and texting a boy all week. she said her dad said it was ok. but at 14 she knows better. she doesnt get the phone back at all now.