Whilst the flaws in Signaling System 7 (SS7) are the gift that keeps on giving, in this case, that gift has been inherited by the DIAMETER protocol, to the delight of miscreants unknown... With internal system, billing and bridging protocols like these, deeply embedded in cellular network infrastructure (all carriers) - who needs enemies; which brings to mind: 'We have met the enemy, and he is us! - Walt Kelly's Pogo, h/t