[Yo-Yo]

This is the first I've heard about President Trump refusing to use a secured phone, so I did a bit of Googling and found this Politico article. Excerpt:

The president uses at least two iPhones, according to one of the officials. The phones — one capable only of making calls, the other equipped only with the Twitter app and preloaded with a handful of news sites — are issued by White House Information Technology and the White House Communications Agency, an office staffed by military personnel that oversees White House telecommunications.

While aides have urged the president to swap out the Twitter phone on a monthly basis, Trump has resisted their entreaties, telling them it was “too inconvenient,” the same administration official said.

So he is using fully secured phones. He just isn't having the Twitter phone replaced every month. Not best practice, perhaps, but not the "deliberate, cavalier use of a cellphone that doesn’t have strict security safeguards" that the NYT article suggests.

MORE FAKE NEWS.

[COBOL2Java]

While aides have urged the president to swap out the Twitter phone on a monthly basis, Trump has resisted their entreaties, telling them it was “too inconvenient,” the same administration official said.

The only reason I can think of for "swapping out" the Twitter phone is to check it for compromises (hacks), and improve hardening (e.g., change passwords, upgrade software). They probably do the same with the other phone.