Better off without them
I ran three PAYG mobiles with them for some time. In the early days, their customer service was quite good, but it is now beyond a joke. The on-line top-up of the PAYG accounts rarely worked first time. Sometimes it claimed that a transaction had failed when it had not. Sometimes it double-debited my credit card.
I went through a phase of receiving bills for someone with a middle-eastern name, but otherwise correctly addressed to me.
As time went on, even threats of the regulator failed to have any effect. In the early days these would result in random calls with promises of follow-ups. These never happened.
Latterly, the chances of acknowledgement of any postal communication were absolutely zero.
As I say, better off without them.