What would it take for someone to spoof a cell phone number

I happen to know for a fact some people I have talked to over the years haven't been the person I was talking to. Would this involve sim cloning or something? This person certainly is not some script kiddie they are government connected(but "not the government".. hint). Anyway, I know I'll never be able to prove it but I'd like to know how easy or not it is.

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Also is this possible with say Imessage on an iphone? Would it come up as Imessage or would it always be "green" as in a "regular" text.

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from hacking: security in practice https://ift.tt/3fIMjSu