What is the ruling on using a company-owned work phone for personal purposes outside official working hours without the employer's knowledge, and is it obligatory to report this if these colleagues do not accept advice?

Using a work phone for personal purposes without permission is not permissible, because it is considered consuming people's wealth unlawfully. If its use is permitted to a certain extent, then it is not permissible to exceed that extent. If the employer's permission for personal calls did not exclude Saturday or Sunday, then there is no harm in using it outside working days. However, if it is known that the employer does not allow this, explicitly or implicitly, then their action is forbidden, and they must be advised and warned of being reported. If that does not work, then it is obligatory to report them if there is no other way to prevent them.