Rob-Hull:
You need to check a couple of things. Under your O2 contract, what are the contract terms about reporting a stolen phone and your liability for consequential loss ie. phone calls made since.
Does your O2 contract include Insurance to cover the phone and associated losses. If so what does the insurance contract state about your responsibilities to report the phone as being stolen, to minimise loss.
Once you have had a good read of ALL information, post back with the details.
You could also contact Consumer Direct once you have all the details. http://www.consumerdirect.gov.uk/
I am sure there is a set procedure for these situations and therefore you should not rush to judgement that you will owe £1100, without a FULL investigation of all aspects.
In the meantime you should continue with the normal monthly payments to O2 and follow their complaints process if you have a problem, including reference to Otelo. I think you will have to pay something, but not the full £1100. I believe there are precedents, where telecoms companies have only been able to make reasonable charges.
Huckster