Trump has apparently given the number to his personal (and unsecured) cell phone to multiple foreign leaders. We already know of one phone call between him and Justin Trudeau about which the staff knew nothing.
But we can be sure foreign governments did.
Remember, it is an unsecured phone.
By now every intelligence service in the world must have the number.
Which means any communications coming into it or going out from it are probably being read in real time by governments both friendly and hostile.
Hillary had a private server that was not hacked, even though communications and computers at both State and DoD were at various times. Comey’s investigation found no evidence that her communications had been hacked, and certainly by now if they had been we would have seen evidence of it.
Now who is endangering our national security by a reckless approach? Who, besides all kinds of other laws he is breaking (on emoluments, on document preservation, on nepotism) is violating the most basic security provisions? Remember, that while Obama continued to have private communications with friends, he used a Blackberry that was secure. I have seen nothing on the public record that Trump has ever secured the device(s) used to tweet and to communicate with others.
Just saying . . . .