Who called from 3103596536

If I do not pick up the phone, they do not leave a voicemail. If I do answer the phone, they instantly hang up. They call at least once a day.
What's the point?

They call multiple times a day.

They've called several times, they asked for my father but I hung up.  There was an echo and it sound quality was really sketchy.

Relentless callers no such number
No message left no one on the othe end
Blocked

Probably a shady collection agency or one of those "free cruise" scams of which they call from many different numbers.

They called me around dinnertime yesterday, and after my machine picked up, I heard a click then a dial tone.

The ID said "Unknown Name" of course.  I never answer calls with ID's I don't recognize.  If I recognized the NUMBER, I would have picked up.  But I didn't so no go.