https://motherboard.vice.com/en_us/article/598xyb/what-is-ss7-and-is-china-using-it-to-spy-on-trumps-cell-phone

On Wednesday, the New York Times reported that President Trump often uses an unmodified, personal iPhone to communicate with friends and confidants as well as two other iPhones that had been modified by the National Security Agency. According to anonymous White House officials interviewed by the Times, Trump has been repeatedly warned that the calls he makes on these devices are not secure and that “Chinese spies are often listening.”

Last year, a Department of Homeland Security report found that mobile devices created a major weak spot in the information security of government employees. In particular, the report cited Signaling System 7 (SS7), a protocol used by telecom companies to coordinate how they route our smartphone data and calls around the world, as a significant threat.