We're having the same problem - for over a week now, although not as frequently as you got it. Hope they piss off soon.

Received a call from this number and it was a friend calling using skype

Got a call from this number.  Nobody there.
Called back, got a not in service message.

Got a call from this number.  Nobody there.Called back, got a not in service message.

Someone has called me from this number twice in the past week. No message is left. I can assure you it is not a collection agency because I don't owe anyone or any organization any money (except my mortgage). If it is a collection agency, then they are calling the wrong person. But, I suspect it's a spammer.

Just to let you all know, this number is a PRE-PAID CALLING CARD.  When someone buys a pre-paid calling card and makes a call it goes through local routing; the local routing uses one way a line as it's not actually an active number, this is why you can't call back and is the reason why it says the number is not in service...because it's not.  As far as the line being dead on the other line, this may be to poor connection on the other end (a cel phone user perhaps).  So folks don't fret next time this number calls you, it's probably just the mother of the illegitimate child you fathered while on spring break trying to collect some child support.  Cherish these moments, having children can be a wonderful experience! ;)