This is one of many atomic clocks in the world, this one is in Colorado. (Coordinated Universal Time)
The number should not be calling you, however you can call it if you want to get the worlds most accurate time for free.
It also has periodic updates on solar flares etc. It is also accessible via Ham Radio.
Definitely not a spammer but if you are receiving calls from it, they are being spoofed by someone else so
I wouldn't answer if you are called from this number.

Recieved call from this number let it go to my voice mail but left no message.

Right concept, wrong agency.  It's actually the number for WWV, which belongs to the National Institute of Standards and Technology (NIST).

This post is not a compliaint, but rather just information. 303-499-7111 belongs to WWV, in Boulder Colorado, which is the most accurate time on the face of the earth. They give the time every miniute.  Its great to use to set your watch or clock right down to second. You can also purchase watches and clocks that recieve the signal to set them selves right down to the second, You can pick up the same signal on a short wave radio on 10 mhz and a few other short wave frequences. This number does not call out. If you recieve a call from  this number, it has been spoofed, and you should report it to the fcc.

This number is U.S. Naval Observatory atomic clock time signal number.  http://hyperphysics.phy-astr.gsu.edu/hbase/acloc.html
Can be seen here.

Must be spoofed from a collector or telemarketer.  It's a number to check the time.

This is the first that I have received a call from this number.  I let it go to my answering service, but they did not leave a message.  If this is a spoofed call then I would like to know who spoofed it.

NIST time of day line (UTC).

See http://tf.nist.gov/stations/sig.html

This number should not be calling you. If so it may be spoofed.