So a random person on Reddit claimed there's about 800 million possible uk mobile numbers, some people have multiple numbers so ballpark 80 million active phone numbers. This gives around a 1:10 chance of picking an active number at random. If there's actual patterns in the numbers this could be even more likely.
What's interesting is this won't have been a realistic sounding number.
Company lines typically start 0300 or 0800 but mobiles are 07... Something.
So if it was just hallucinating, it did so badly.