I was already an O2 customer on a SIM only contract but was being regularly contacted by O2 to upgrade. I researched the phone I wanted and found that mobiles.co.uk offered that phone on O2 for a very competitive monthly tariff. The next time O2 contacted me I quoted their price but was surprised that the best tariff they could offer was £28 per month compared to mobiles.co.uk of £18 pm. I phoned mobiles.co.uk and asked if the offer was for new customers to O2 only, but they said it was open to present users as an upgrade and the number would be updated automatically. I then went to an O2 shop and again asked for an upgrade price, but again they said £28 and I would be best to go with the £18 contract I had been offered. I then purchased online. Within a couple of hours I had an email response from mobiles.co.uk to say that financial details I had supplied were in error and I should log on again to correct them. This I did, but working in the computer industry I knew that the data I had originally supplied was correct. This was later confirmed when I checked with Equifax which indicated that a credit check had been made on me that day. Three days passed and I heard nothing more so I phoned mobiles.co.uk to find out what was happening. My order was apparently on hold because the final digit of my credit card was apparently missing, even though all my data had been re-entered. After some time on the phone I was eventually able to get the order processed. The phone was delivered on the day I requested and was in perfect new condition and worked well. However, when I tried to update the number through the 'My O2' app I only got error messages and could not update to use my original number. I then had to phone O2 to sort it out. O2 were very helpful at this point and explained that I had purchased a new contract and not an upgrade contract so I effectively had two separate O2 accounts. They cancelled my original SIM only contract and all was sorted. To summarise, mobiles.co.uk provide phones and contracts at unbeatable prices but are less good in my experience about data handling/processing and being transparent about what is a new contract and what is an upgrade. However, I am very happy with my new £400 phone which costs me only £6.70 per month more than I was paying for a SIM only contract which had only half the data and minutes allowance.