Had a missed call from this number (+12128640943). Called it back and there's a message that says "The number you called is not in service". Well, there you go!
I had the same number call just now 15.00 BST. Didn't answer as our IT consultant says they are spam generated calls spoofed to look like they've come from USA but actually originate elsewhere - his guess was Ethiopia as that seems to be the current favourite.