who are mdr and why are they phoning me?

Answer 1

If the message they leave is too obscure to work out what it's about, chances are, it's a SCAM.

If you have not signed up to the Telephone Preference Service, do so now.

Try 1471 to see if they have a number - is so, you can ask your phone provider to block that number - if not, then you could ask them to block all 'number withheld' calls (I expect they will charge about £1 a month to do so).