Why won't the White House security experts set President Trump up with iPhones using the Signal app?

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?

How do you know thd Signal app wasn't written by North Korean hackers and sends everything to Lil' Kim?

I wonder if they've tried… And he's refused to use it.