There's two means of number identification -- caller id (CID), and automatic number identification (ANI). ANI used to be a lot harder to spoof than CID; likely because ANI is always delivered out of band, and some early CID spoofing was done by just overplaying in-band CID. On the consumer side of things, you're allowed to disable CID on outgoing calls, but not to disable ANI.

Toll free routing may be less amenable to grey routes and things as well. International callers aren't "supposed" to be able to call US toll free numbers, which may make it harder to get to. I've seen some companies claim that they can use call routing information to toll free numbers to get accurate caller id information in some countries; but I would never trust it.

You also shouldn't trust the source IP, or the return address on a standard envelope in the mail, unless you have convincing evidence. It's hard to think of an example of a source address in communications that's really trustable.