[andymc]

I think this may be a result of malware or even ransomware on your system. Don't ring the number at all. Go to filehippo.com and download MalwareBytes, then run it and see what it comes up with. Ditto CCleaner, if you don't have it. EDIT I see Gren_24 has provided the direct link for the first one.

You haven't said what sort of system you are running, what browser you're using etc but hopefully you have Windows Defender.

The way a lot of malware, trojans etc access your machine is by "spoofing", i.e. sending out what appear to be legitimate emails in the hope that the recipient will click on a link that then installs the dodgy program. For example, I occasionally get emails with the title "Receipt for your purchase from the iTunes Store" (even though I have never bought anything from the iTunes store!) or "There is a problem with your Paypal account", with a subsequent instruction to "Click here to contact us". Or perhaps something purporting to be from your bank, especially if you actually do use online banking.

Never click on any links, attachments etc in such emails. Usually, if you hover your mouse pointer over the sender's email address (WITHOUT clicking on it), the actual email address will appear and you will probably be able to see whether it looks kosher or not. Either way, if you think there may be a genuine issue to be resolved then you should contact the company directly, not via the email they appear to have sent you.

If you haven't backed up your system already and/or made copies of important files & documents that are stored on a separate drive, I strongly suggest you do so as soon as you can.