Call went to voice mail, no message left, probably a charity

Caller: Wanting to do a questionaire on behalf of an unnamed charity.

I said I was in a hurry as I was on my way out. She said it would only be a minute, but I said sorry, had to go and hung up.

Unknown number. Went to monitored message machine. Wouldn't leave a message on the machine. Checked number … recorded message "The number you have dialled is not in service"
Probable scammer

No immediate reply when I answered so I hung up. I could hear noises in the background like a call centre.

Answered call - no one replied - then a dial tone, someone said something just as I was hanging up - sounded like a call centre in background

Same as the person before me. The person phoned me on my landline, at 6:34 pm on Saturday, 20 October 2018.
When I answered, they put down their phone.