Second day in a row and do not leave a message

Got a call from this number, let it go to voicemail but no message was left. Called back and heard the beeps that Susan described but someone's voicemail message about how she was unavailable and couldn't get to the phone so leave a message. Did not recognize the voice as someone I know.

i answered hello, but heard strange beeps then a long pause then i think someone eventually picked up the line and said "HELLO?" just as I was hanging up myself, and they didn't call back. I have no idea what this number is or why they called. Weird.