Cancelling Softbank Contract

Today i went to Softbank to cancel the contract which was made 6 days ago but I was told that it cannot be cancelled without any explanation

For context my mobile phone broke 2 months ago and I sent it for repair. The phone got lost somewhere in the process after it was sent. Since Im a university student, i didn't have enough savings to buy a new phone so made a contract with softbank that gives a basic smart phone for free. 2 days after signing the contract, i received an update that the phone was found so now I don't need the softbank contract.

I understand its my fault for signing in the first place but i want to know how long do i have to wait before cancelling the contract

by ThroughTheWire67