If the money was improperly taken from your bank account by the former cell phone company where you in fact did not owe any more money on the account, you should immediately call your bank to contest the debit and follow up with a written letter to the bank objecting to the withdrawal if in fact you had paid off the debt in full. By doing this, perhaps the bank may be responsible for an unwarranted and improper withdrawal from your checking account.However, it the former cell phone company mistakenly told you that your bill had been paid off in full and it was not, then what you were told was done mistakenly and the debit from your checking account albeit a surprise to you was allowable.