Are fake phone number apps legal?
There are many apps that enable you to obtain virtual phone numbers (we make one of them, Burner). Phone numbers that are issued via these apps are generally being issued or hosted, at some point up the chain, by regulated carriers and are perfectly legal, though the specific regulations vary by market.
There are other apps that let users "spoof" a number -- ie call someone and disguise it as a 3rd-party callerID. It is illegal to do this in the US, and reputable carriers and intermediaries generally have protections in place to prevent this from happening.