Sometimes it arises from the nature of the enterprise--you wouldn't want to have to have dozens of different telephone lines in your house just so you could connect with all the different phone companies your friends might select.
It doesn't make much sense to have many phone services for all your needs.
um-hum yeah see i have an unlisted telephone number but i still get all of those calls and then some of them are speaking in a foreign language that i don't even understand so yeah i do i really feel that that's uh an invasion of my privacy i agree with you on that particular subject there let me see