Help with my rights for faulty phone please

Olddad

Hi all. My teenage son bought a phone from T-mobile a few months ago. It became faulty after about six weeks or so. He took it into the shop and they told him that they would repair it within 28 days. It has now been 28 days and it is not yet mended.

Unfortunately I don't live with my son, he lives about 100 miles away with his mum (who is no good at dealing with this sort of thing). I will be traveling over there next weekend and will be going to the T-mobile shop to try to get him a replacement phone, rather than wait for an unspecified time to get back a repaired one (after only a few months).

Can anyone tell me how best to prepare for this. I need to do it in 'one hit' as I can't just nip back if the manager isn't in or something? What are my son's rights please? What can I quote at the staff or manager that will get me my rights, on the day.

Thank you.