I would suspect that the main reason is that they are using a third party service to call customers. Since the third party service is hosting for multiple companies, they don't put in a company name in the outgoing caller id. Some of these services are noted for hosting for some shady companies as well, so I would suspect that they frequently rotate DIDs, (phone numbers) to bypass phone blocks as well. It really does suck, since running a VoIP server is pretty cheap, and not much maintenance for an IT staff. If they ran their own service it would probably save money, and fix the issue you are having simply by filling out the caller id name. Given this is the same reason you might receive a call from the IRS that is originating from some other country, but that's another issue.
I frequently get annoying phone calls from numbers I don't recognize to my smart phone. Some of these calls are legitimate from large companies like CVS and companies I do business withm but the shown display only shows the phone digits so I don't answer. Why can't large companies place their names on the phone instead of just a phone number? What percentage of cell phone users still use non-smart phones at this point? Thanks.
--Submitted by Phil R.

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic