https://www.techdirt.com/articles/20191114/10274143391/impeachment-hearings-highlight-more-trump-phone-opsec-failures.shtml

Plenty has been made of the President's unwillingness to adhere to anything close to reasonable security when using his mobile phones. Whereas the Defense Information Systems Agency (DISA) and the National Security Agency usually work in concert providing state leaders with "hardened" devices that are heavily encrypted, routinely updated, and frequently swapped out, Trump has refused to use these more secure DMCC-S devices (effectively a Samsung Galaxy S4 device utilizing Samsung's Knox security architecture) because they apparently infringe on his ability to Tweet.

Just a few months ago, Senators sent a letter expressing concern that Trump's mobile phone practices were leaving the President open to potential hacking by foreign entities: