security researcher here, but cellular protocols are one of the few things I hadn't touched, only slightly faniliar from reading and hearing. so reading your ti

security researcher here, but cellular protocols are one of the few things I hadn't touched, only slightly faniliar from reading and hearing. so reading your title made me wonder what actually can be done, aside from everything people wrote here. seems like this is a gap in security, globally. it really seems to be (one of) the weakest link(s). all other answers are fine, under certain conditions, impractical in reality. one of the things I do think can be an improvement is using virtual sim, such as twilio, then it's more manageable, get notifications, probably more secure than employee being socially engineered to give your number away...