In this NY Times article they say the Presidents iPhone calls to his friends are not secure and the Chinese and maybe the Russians are monitoring them.
Why don't the security experts install the Signal app on the President's phone, and tell his friends how to install it on their phones and then ask them to call using Signal?
Read more: https://iphoneus.org/faq/why-won-t-the-white-house-security-experts-set.html