These [***] just keep calling and they don't leave a message.

Like Brandon above, they call every day and do not leave a message.  One time you could hear an office setting in the background and then someone said the name, "Mike Chad" and that was all

Received a call from this number today.

A male with a thick Indian/maybe Bangladeshi accent said he was looking for our office manager.  I advised him that corporate policy required I find out his name, company and detailed reason why he is calling.  He said his name was Nick and was calling from XDBS and wanted to talk to her about our phone system.  I told him he is not one of our listed vendors and I would need more information about why he is calling.  It sounded like he said he was calling about a code.  It wasn't until we went back and forth I realized he was trying to say quote.  
I attempted to advise him that our corporate policy was we do not accept telemarketing calls and that if he had any plan, proposal or offer, he is more than free to send it to us in hard copy to our address on file.  And that if he is unable to do that, to remove us from his call list.  I suspect he hung up during that.

Interestingly enough, there is a XDBS on face book with a number very similiar to the one above.  The profile says they excel at the following:

Data Cleansing & Contact Discovery
Qualified Sales Opportunity Creation
Account Profiling & Infrastructure Mapping
Events & Webinar Registrations
Email Opt-Ins & White Paper Subscriptions

Basically a mass-marketing clearing house.

I receive a handful of calls from 301-566-0000 every day on my desk and cell phones. Even if you answer no one is there. Must be a machine.

I get a call from this number daily. They usually have an Indian/Bangladeshi accent while claiming they work from IBM and would like to send white papers on a particular product or service that they are offering. Upon multiple requests to stop calling me, they continue to do so. I have had to block their number from my cell phone as well as phone system.