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The question is already in the headline:

Why is callerID spoofing not possible when the destination number is a short code?

Here for example the answer to the question if callerID spoofing is still possible says "It is possible to do so with a 10-digit code so that it looks like another phone number but NOT from a short code. The way that short codes function in the U.S. prevents that from occurring."

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Short codes have behavior similar to 1-800 numbers for phone calls in the US. These provide the original number because the destination is being billed for the call, so the carrier provides the real number as part of the service. This is provided by a different system than Caller ID, called Automatic Number Identification.

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