It's likely that the device has been locked down for security reasons. Trump is previously believed to have used a Samsung Galaxy S3.
White House officials tell Axios that key to getting a more disciplined Trump is to limit his screen time which means less watching TV and banging out Tweets.
Trump himself has been pushing staff to give him more free time. But staff does everything it can to load up his schedule to keep him from getting worked up watching cable coverage, which often precipitates his tweets. It has worked well overseas so far.
Trump's extensive use of Twitter is helping struggling service, according to BTIG analyst Richard Greenfield.
"It is undeniable that Twitter has been thrust into the global zeitgeist following the U.S. Presidential Election in November 2016," Greenfield writes. " Trump is giving consumers a reason to learn how to use Twitter, furthering the reason for public figures etc. to put content on the platform. Twitter is in effect getting a second chance to attract and retain users."