If you call the number back, you get an automated message.
"This is our opt out required by the fcc" (or something, not exact)
Input your number and then it repeats your number back to you and tells you it has been saved.
To what, I do not know. Would be magical if it really was their "Do not call" list
alternative formats for 303-532-4283
(303)532-428301113035324283(0111)-303-532-42830111/303 532 4283(0111) (303) 532-4283+13035324283+1(303)5324283+1303/5324283+1303-5324283303.532.4283303/532.42833035324283