They are all as bad as each other
They are all as bad as each other.
I've ported out (and back into) Three a couple of times, as they try to fuck over existing customers after the minimum contract term ends and unjustifiably push them onto much more expensive tariffs, with no way to switch to the normal tariffs on sale to new customers (which are comparable with, or often cheaper than, the existing contract). It's much easier to port out to a 30-day SIM-only account with a different network, and then later port back in as a new customer than have to waste time fighting with the call centre over this ("Never send a human to do a machine's job."). It's highly frustrating, because if Three didn't dick around with existing customers, I'd generally be perfectly happy to stay with them. The usual big company problem of completely failing to recognise loyalty.
This results in them having to refund some of the last month's payment after the port-out date, but rather than just refund it automatically via the direct debit mechanism, they send a paper letter asking the (ex-)customer to call them to arrange the refund (with no option to do it online via their absymal webshite customer account management area).
I'm sure this is deliberate, as not only is there a risk that someone might have moved (often a reason for porting, if the network is not available at the new address), but the hassle of trying to call the call centre and wait in an interminable queue means that I'm sure that few people deem it worthwhile for a couple of quid. If they really can't refund via the direct debit mechanism (which I very much doubt), they should at least notify customers via the mobile number and/or email address that they have for them, as well as a letter, and allow the refund request to be processed via the website.