Mondo ******* contact activation

During lockdown my son who is a minor answered a call from Mondo. Short end of the story is that we are now the owners of a new 3 year contract. When disputed they refuse to cancel the contract. There is a case number with the recording and although we keep telling them they did not speak to my husband they refuse to listen. There was none of the normal varification questions asked as according to them they do not need to as they have all our information already. We have asked them to compare the voices when speaking to my husband with that of my son but to no avail. We have contacted Vodacom with whom the contact is but because Mondo contracts for them they can not cancel without Mindo giving the go head. Any suggestions as this is now the 3 month we have to pay for something they sold to a minor who is not the owner of the contract?