(CNN) President Donald Trump is increasingly relying on his personal cell phone to contact outside advisers, multiple sources inside and outside the White House told CNN, as Trump returns to the free-wheeling mode of operation that characterized the earliest days of his administration.

"He uses it a lot more often more recently," a senior White House official said of the President's cell phone.

Sources cited Trump's stepped-up cell phone use as an example of chief of staff John Kelly's waning influence over who gets access to the President. During the early days of Kelly's tenure, multiple sources said, Trump made many of his calls from the White House switchboard -- a tactic that allowed the chief of staff to receive a printed list of who Trump had phoned. Kelly has less insight into who Trump calls on his personal cell phone.

While Trump never entirely gave up his personal cell phone once Kelly came aboard, one source close to the White House speculated that the President is ramping up the use of his personal device recently in part because "he doesn't want Kelly to know who he's talking to."

The senior White House official said Trump "is talking to all sorts of people on it," noting Trump's barrage of private calls is a "recent development."

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