Called 10x in 23 minutes

This number called me, but when I answered, it sounded like their end was muted. After about 10 seconds, the line disconnected.

I answered and they hung up.  Tried calling back but the call could not be completed as dialed.

Missed the call, tried to call it back but got a message saying # could not be completed as dialed.

The number is still being used. Hung up when I picked up. Tried calling back from a different voip number. It rang 3x then aborted. My hunch is it's a robodialer sniffing for active numbers as subsequent targets.