Towing Issues: question about towing, tow truck company, tow company

Question
my son parked at an apartment complex, went to school and when he got out his car was gone.  The manager of the apartments told us she observered him parking there and called the tow truck.  My son's friend parked there and was towed as well.  That day I went and paid the fee and got his truck however his friend's father went to get their car and told the tow truck company that they did not have a permit number on the no parking sign so they could't charge him.  Is this true that if a permit number is not on the sign then they cannot tow you?

Answer
Since the locality wasn't mentioned the question can't be properly answered. If they were truly able to retrieve their vehicle without payment for trespassing my suggestion is follow his recommendations. If the tow company was in the wrong you should be reimbursed. As I said, I can't say because more information is needed. Good luck.