I've received 10 calls from that number over the last two days, I now see as I look on the caller ID of the landline phone I rarely pay attention to.  When I call them back, I either get a busy signal or a message that says "...is not available."  

I'll try to answer next time they call and see what happens.

Dead air... and a message says "..is not available" and hangs up on you. Is calling at verious times in the day for the past few days.