Call and never leave message blocked and delete

Called on mobile and didn't leave a message. No idea who they are.

heard my message and hung up, seem to have a lot of numbers in those sequence's, used for annoying ppl

Caller: unknown

Called me at the usual time for anonymous calls - late afternoon. I didn't answer, checked the number here and found lots of other people who had received calls from it. Blocked it.

No message left can’t be serious.

Doing market research for politics

This number called me, I didn't answer because I didn't recognise the number. No message left. This number is related to 80+ other numbers from the same people, always the same response when you call back. From all different states, and sometimes mobile numbers. Always the same switchboard. I have blocked nearly 90 numbers, I believe they are debt collectors but I owe nothing to anyone so it must be a scam.