They are only exempt until you tell them to remove you from their calling list. Any calls you get after your request is in violation of the Do Not Call and subject to penalties. I verified this with the Federal program. Tell them to remove you and the next time they call and tell you they are exempt, tell them they need to better understand the law and call the Federal program to verify.

after reading some of the comments, I looked up both Heifer Int'l & MDS Communications.  It's not a toll free number listed on their websites

The Dave and Mara listed below are part of org.
Scams
They also post on here that it's a legitimate fundraiser .......for them!

That's why they don't talk or leave message!   They want you to answer so they know its a working number

They are a scam.   They are a professional fund raiser for themselves.
Fire, police, reputable charities, Salvation Army , st Jude. Do not use these methods
they just want your number to resell.  When you answer or contact them, you let them know you have a working number.
Do not give them any info. Don't press 1 or whatever.   You will prob get more calls now.   Don't answer, don't call back

Also political and fire and police do not do fundraising like this

Scam.     They wanted your number

I keep receiving a call from the above number.  I want it put on the do not call list please.  Thank you.

They are fundraisers for various organizations. I called them back and heard an extremely polite voice message that offered me the option to place my number on their do not call list.  Which I did.  

I got a call to but I didnt answer at all

Constant nuisance calls that NEVER leave a message...so tired of it!!!

no msg. & I won't answer nbr. I don't know! I thought no-caller list was suppose to stop tell markers , etc.!

Receive calls from this number REPEATEDLY!!!!! Just went to their website to "opt out". We'll see what happens. Until then, will go on my call block list.

Did not pick up and caller left no message.

Got a call from this number on Saturday 5/10/2014 at 2pm. Did not answer. They did not leave a message.

When they called, I plugged my phone into a nearby electrical outlet and heard the person on the other end of the line start screaming. Uh-oh.

I received a call from this number 888-207-1231 on 04/24/2014 3:00 PM on my cell phone. No voice-mail was left.

Ugh.  Since last October, I've been bombarded with phone calls from charities.  I think it started when a neighbor on my block collected for the American Heart Association.  (Her husband did die from one).  I donated and started receiving calls from AHA (Heart Assoc). Asked to be removed from their list.  Then Breast Cancer, Veteran's Fund, Children's Cancer Fund, Ronald McDonald House, Pro Life Campaign, Republican Campaign, etc. all started calling me!!!  I don't answer any (888), (877), (213) area codes or 800 Service numbers.  Now, "Touchtone" is showing up on my caller id.  STOP CALLING ALREADY!  

As this appears to be a middleman that raises money for various charities, etc.  this is almost certainly a scam preying on those who want to send donations to charities to redirect the money to themselves.

Received call, went to voicemail, no message.

Beth, NO ONE can get any information from you (bank, credit cards, etc) just from you picking up the phone. It is just NOT possible. All they can tell is that someone answers the phone at that number. Nothing else unless you give it them.

Yet another "middleman" begging for donations for different charities.  SCAMMER!

Another scammer wanting money without the guts to identify themselves or even leave a voicemail.

dee,
I don't know if you'll read this, but if you do, I'd appreciate it if you'd let us know if this number calls again representing Habitat.
I'm a long time supporter of Habitat. My father was. Still is. I have been. My people have donated thousands of hours. We've been involved with the construction of 15 homes in three counties here in WNC.
I would dearly love tracking down a bogus operation using Habitat as an alias for fraud. I hate frauds. One using a cause that hits home would put me in to a state that would set a fraud running with about 2 dozen well armed construction workers breathing up his a**. With very bad attitudes. I kid you not!
Habitat for Humanity was always a special program for us. It fell apart. That doesn't matter. If some frauds are trying to use their name, I want to know about it.
Live Well.
Gunnar

Said they were calling for Habitat for Humanity. Since I don't donate over the phone I simply hung up. There is no sense in asking these companies to stop calling. They don't respect that.

MDS communication calling for St. Jude, a not for profit org..

Soliciting calls from unknown.

I got a call from them on 2-27-14 @4:27 pm and usually never answer calls I don't know but was waiting for our realtor to call and thought she may be on company toll free line so I said hello no one answered. So I got worried with all the calls I have been hearing that all you have to do is pick up and they can get your bank, credit cards, etc info in a second. So I called back on cell and it said charity, but I blocked on my home phone. Man I never pick up phone with so much scams going on. Thanks for the info.

We are on a DO NOT CALL list. Tired of getting these calls that don't identify themselves.

You can call them.and and use their "opt-out" system but it will do you no good. They will call you back from another number at a different location. This one was San Bernidino, CA. The next call came from a totally different number from the L.A. area near a lake, according to the map. If it's your mobile phone add them to your call rejection system.