Bridge security is a joke if there isn't any way to moderate it. Even if you're exclusive to whom you give the number, once someone who isn't trusted gets it it's gameover, go find a new room and repeat. Not to mention there are only so many room numbers, so they can be scanned (but that would take a HELL of a long time).
I like the way teleconferences are set up with an admin login and a user login, all based off of an 800 number. *shrug* That would be the ideal situation. But that's never going to happen :/
Moving providers would work as long as there's some way to moderate it, otherwise it's just picking random room numbers and hoping no one else finds it. Security through obscurity doesn't really work, or only works for a short while.