If you are wondering if other countries are intercepting calls by hacking the carriers, the article seems to mainly point the finger at malware. It said the protocol is to replace the phones with no backups every month or so.

"Mr. Trump is supposed to swap out his two official phones every 30 days for new ones but rarely does, bristling at the inconvenience. White House staff members are supposed to set up the new phones exactly like the old ones, but the new iPhones cannot be restored from backups of his old phones because doing so would transfer over any malware."