Same here.  Looking at the call logs I didn't delete, they average 8 calls per week. They seem to call in waves, back in 2012 is started in June, then dropped off, in 2013 it was March, and this year it has started in April.  The number is a VOIP spoofed to Roscoe, Texas (it's small enough that everyone knows everyone's phone number) serviced by Level 3 Communications, LLC.  Because of the seasonality of the calls, I'm thinking it might be the DPS or Sheriff's Association fundraiser; then again it could be some scammer.  When my voicemail picks up, they hang up and almost immediately call back and hang up before the voicemail picks up.

The Level 3 General Manager for North Texas is:
Chris Conley
855.381.0413
972.914.4944
19111 N. Dallas Parkway, Ste. 225
Dallas, Texas 75287

To file a complaint with the FCC: http://www.fcc.gov/complaints

Calls me at work several times a week. No message and usually no one on the line. Today I heard some brief conversation between two workers which was undiscernible.
Although the area code is the same as mine I suspect it's a spoofed number. Just once I'd like to actually talk to a person and "try" to opt-out (along with a few choice words!).