Calls but hangs up before I can answer.

Maybe a hacker

Had a missed call from this number on returning call, was told that it was some group wanting to compare my electricity and gas accounts, pressed 1on request but on answering I was left hanging on the line with an automated voice telling me that someone would answer shortly which after a couple of minutes was informed that no one could take my call at the present time, had a number of calls like this from same group, I have blocked a number of times, with these people calling back on a different number.