Keep calling every day. Hang up before you answer

I let this unknown caller go to voicemail.  The message was a lengthy "blank" before it was disconnected.

Somebody called me from this number twice.  The first time they called, I wasn't able to pick up and it went to voicemail, however when I listened to my voicemail, the message was blank.  Person called from this number again 3 minutes later and this time when I picked up, nobody was there.