I had 30 unknown callers for this month. Kept blocking them but they kept calling .

Rang me several times, never leave messages, and when you call them they never answer.

Has rung several times but left no message so I assume it's another rubbish call.

Comes up as suspected spam on my phone. If you call back you get the engaged tone.

Has called several times, but doesn't leave a message.
If I cut them off, or if I let it go to the answering machine, they know it is a real number. Which is best?

Phone identified it as potential spam, blocked

Hung up when I called back and didn't leave a voicemail when they phoned. I am blocking the number.

Suspect it's a nuisance call, no message left

Rang but left no message - rubbish call no doubt