There is (or was) no authentication within the core of the public switched telephone network, since it was designed at a time when that was impractical and physical infrastructure was assumed to be reasonably secure. So you don’t need to fake signing, you just say “Hey, +1-555-555-5555 roamed onto my network and is making a phone call” and the recipient takes this at face value. (“Blue boxing” to fake the phone system into giving you free long distance phone calls worked for similar reasons.) STIR/SHAKEN is supposed to fix this, though I don’t know how far along implementation has actually gotten.