You can sue your ex for breach of contract for failure to pay the phone bills.You can either seek damages (monetary compensation for the amount owed on the phone bills) or specific performance.Specific performance would be your applicable remedy (instead of damages) since you want your ex to pay the phone bills.  Specific performance means the court judgment would require compliance with the terms of the contract (payment of the phone bills by your ex) as he had agreed.You may be able to file your lawsuit for breach of contract in Small Claims Court.  If you win the case, you will also be able to recover court costs.  Court costs include the court filing fee and process server fee.