They keep calling but don't leave a message.  Who are these people?

They have called my house numerous times and either don't leave a message or hang up when I answer. Just today I answered and it was a solicitor trying to sell me tickets to some childrens program, and when I said no thank you, they then proceeded to grill me on why I didn't want to buy tickets.

The AT&T *60 call block feature works with this number, at least in the Dallas area (where the 972 area code is located).

I received calls from 972-253-2105 several times, they didn't leave any messages and one time, when I picked up the phone, they hang up on me!

I've received calls from this number and no one talks, just hangs up when I answer...and I let the phone ring several times, so I know they do it on purpose.  What's the purpose of calling if they don't leave a message or talk????