Caller: Scammer

Silent call. Could clearly hear my voice feedback (repeating whatever I said). If you mute the call, the line is (almost) silent except for static.

I am almost certain the caller/caller's equipment is recording the receiver's voice (saying "Hello") for malicious purposes. They will likely use this to bypass voice verification for fraud purposes.

OR they are numbers and verifying they are active before they proceed with identify theft or other fraudulent activities with your number.

If you receive a call from this number, do NOT greet the caller or say your name otherwise you may be a victim of fraud.