[T-hawk]

You could do that, but it would be a terrible idea and experience in practice.

1. Your system is unusable from anything that doesn't provide an accurate and usable caller ID number. Such as any office where outgoing calls route through a trunk number. Or many VOIP services.

2. You've quite possibly doubled your phone bill, or at least substantially increased it.

It's not always correct to throw as much security as possible at a system. Security always involves tradeoffs and sometimes it is correct to make them.