What is the ruling on using a free balance from a telecommunications company after leaving employment with them, knowing that the company has not responded to requests to cancel it, and that it prevents the use of the primary balance?

If the company mistakenly transfers credit to your mobile phone, you must stop using it. Your action of refraining from using the number with the credit is correct, because it is not permissible for a person to dispose of the property of others without their permission and consent. A clear error is not to be considered [as a valid transfer].