It is not the 1234 at the end that is suspicious, it is the '303-720' that looked suspicious even before I looked it up to confirm what I suspected.303 and 720 are overlay area codes for the same place, the Denver Colorado metropolitan area. No telephone company would assign as prefixes, codes which are used in the same or an adjacent area simply because the possibility of confusion would be tremendous.
I live in Northern Virginia, which has both 703 and 571. Nearby is Washington, DC (202), South and West Maryland (301 and 240), and south virginia (540). There is not going to be a 703-571, a 571-703, a202-703, a 202-301 or any similar combination because of the potential for confusion.
Basically anyone that has their own PBX can program the caller-id number generated for outgoing calls. I routinely get calls from companies that either only display their toll free (800/888 etc.) number, or generate a fake number like 999-999-9999 or 000-000-0000.
This can be done with some VOIP providers as well, depending on how they set up their systems.
Further, as has probably been mentioned here, you can purchase a calling card for 10c a minute that will allow you to call into an 800 number and set the originating number on calls made using the card, so you can give any number you want as the caller-id value.