I am an existing O2 customer through carphone warehouse. I called customer services on above number to cancel my contract and was transferred to loyalty department. They offered me the above deal. They are also more relaxed about sending the bills for cashback. Should be months 6, 12 & 18, but you could send all of them at month 18 and you will get your whole cashback.
The contract is actually £30/month for 18 months with 3 x £150 cashback.
So 18 x £30 - £450 = £90 for 18 months
I have used them for a while now and never had problems with cashback unlike some other companies.