According to a recent report from the New York Times, President Trump is ignoring the standard White House protocol of using landlines for telephone conversations. The President apparently has 3 iPhones, two of which have been locked down and made 'secure' by the National Security Agency, while the third is your regular stock device, which apparently the President's using because it's the only one that will allow him to keep a contact list.
The report raises serious alarms in regards to both the President's understanding of security protocols, but also the mobile telephony protocols, through which Chinese and Russian spies are allegedly spying on the president's mobile calls. The frail and insecure state of mobile telecommunications protocols, such as LTE (4G), VoLTE, Diameter, and SS7, all known to contain many known vulnerabilities.
Those very same protocol flaws have been used in the past by US intelligence agencies to spy on foreign officials, such as German Chancellor Angela Merkel, according to documents leaked by Edward Snowden which explains why White House security protocols view landlines as more secure systems.
As expected the President in response to the report tweeted "I only use Government Phones, and have only one seldom used government cellphone. Story is soooo wrong!”.