Was that on a mobile or landline phone?
There seems to be another one going around to watch out for. My wife had a call from an unrecognised mobile number (private number) - she didn't answer - then a couple of texts along the lines of a caller has ran out of credit & asking for her to call a certain number back.
I did a google & found it to be an old scam, whereby if you call said number your bill is billed a high amount of ££s. She forwarded to info to the o2 scam reporting number, & she's not had another call or text thus far.
I mention it because some members on here, or people they know, might have had the same thing happen, so a note of caution really.