Sometimes they have just installed an image with windows style graphics that repeatedly pops up.
So it is far les clever than it appears.
I'd try to boot into safe mode then see if anything has been recently installed.
Do NOT ring any numbers or whatever it wants you to do. Even if you had to re-install the system it would be better than doing that.
I was horrified someone I thought more sensible paid them to 'unlock it' and a couple of weeks later they turned it on again and he paid a second time.