Cancel contract

I am trying to cancel my contract with NOS due to moving away to a new country. I mentioned this in the shop and later that day i received a call. My portuguese is not great but i can follow a normal conversation. They told me that moving away is not a valid reason to cancell the contract and i should tell them who will move into the house so they can change the contract to their name. I have no idea who the next renter will be and when the landlord will find something, I just want to cancell the contract. How is moving away not a valid reason to cancell the contract???