A/C company calling even though I'm on the do not call list.  

They called at 12:53 today March 18, 2014
I answered after 2 rings and they hung up without saying anything

They called at 12:23 pm on March 22, 2014 and 11:09 a.m. on March 1, 2014.  Both times they hung up when I answered.  

call the number back and tell them to take you off their list.  they are very nice about it and it works!!

No.

DO NOT call the number back.
If one is on the Do Not Call list, yet gets a call from a "telemarketer", that "telemarketer" is breaking the law.
Your callback merely confirms your phone works.

(Interesting that somebody found this thread, and said calling back works.  Why look here if it really worked?  Kinda looks like a shill stopped by to leave a deposit.  Also note that the company was not named by the shill.)