Why was I charged?

Long story short, my ex best friend had an account with Verizon and added a line for me to be put on her account. So I paid for the initial costs for my new Samsung Galaxy S7 at the time and paid my bill accordingly. I decided to move to a different state because ex bff and I lived together and I didn't approve of he lifestyle choices. She didn't like that very much so I had to leave one day while she was working. She then shuts off everything with my phone account. Basically I have a phone with no service. So I sold the phone I paid for and a few months later I get pulled over for speeding and get arrested and spend 4 months in jail with never being in jail before. I had to admit to being guilty just for a reduced sentence or I would have served 6 months. How in the world was this possible? I honestly feel screwed by our justice system.