Civil issue

Unless your ex intended the use of a phone on his plan to be a gift you may be legally obligated to pay him back...if he chooses to treat it as a contract between him and you he probably has a good case to force you to pay him back.  You paying him the part of the bill on any regular basais may be enough to form a contract whether or not anything was ever put in writing.  So, the short version is:  yes, he may be able to sue to collect the bills he paid for you so the ball is in his court so to speak.