This number called me today, wasted my time by staying on the phone.... but eventually worked out the company and what it does.
The company is called 'Mobile Phone Warehouse Wales Ltd.'
I believe they call random numbers, con the person into believing that their contract is about to end, and set them up with a new contact, in my case with 3. They will then set you up with a new contract, so you end up with your previous one, and a new one.
My advice is to not even answer, or pick up and tell them your contract is not about to end, do not give them your bank details, when i told them they weren't getting my bank details they lost interest and hung up!