Eventually it was going to hit me and today was the day. The phone number looked too strange to pick up (07077739124), so I let it slide to voice mail. 

What I heard was the famous IRS scam that has been drifting around for the past couple years. At first, it scares the heck out of ya, but realize it's just a scam. The best idea is to report the number to the FTC. Give them phone number on your caller ID, and any other details of the call. 

By all means do NOT call them back. You are not being sued by the IRS, and if they have a problem with you, you'll get a letter.