That is a shocking experience. Although I have always been pleased with o2 service, I have never lost a phone, so I don't know what line they might take. I would have expected a better reaction than that, to suspicious calls.
I wonder if there are any laws against using a phone you have found, for more than to find out who it belongs to and call them? The virtual laws generally are pretty vague but I feel that the only way of improving the system is to complain officially.
After all it is impossible to be so perfect that you never have anything stolen, however careful you are.
I wonder if the top up as u go system can stop this from happening. There seem to be good deals. But I don't know if that prevents someone from making further calls?