Thank you both. I did google it but it looked like I had to dial 33, I didn't realise that would be the number I would see if someone in France rang me (don't want to inadvertently answer the phone to someone wanting to fix my computer problems).
Some phone systems inject false numbers into the system but I'd normally associate that with Indian call centres (wanting to charge me for 'fixing' my computer) than with legitimate overseas networks.
I hope that nobody in Bedford (where the dialling code is 01234) has the number 567890, otherwise they'll be getting a hell of a lot of people (who've had missed calls from that French system) phoning them up!