I bought an o2 upgrade deal through mobiles.co.uk in order to upgrade my pay monthly sim-only o2 deal to a 24 month pay monthly contract with a new phone. Buying the upgrade via mobiles.co.uk went smoothly and the text on the website assured me that everything should go smoothly with the upgrade via o2. Sadly this was not the case and getting my new phone/sim-card activated and transferring my old number to it became a bit of a nightmare involving many phone calls to o2 plus a good few live chats with o2 and being told varying stories that I needed to do a sim-swap but then eventually being told that I needed to cancel my old o2 contract and have my old phone number transferred to the new contract I had taken via mobiles.co.uk. This was not the smooth 'upgrade' that I had hoped for and I'm still not convinced that everything has gone through 100% the way that it should. Time will tell when I receive my first new bill next month and see if I am charged the correct amount or maybe charged for two contracts or something. 4/10 - could do much better.