Cellphones can be "hacked", yes, particularly if she downloads a lot of apps for amusement. But what, really, is the point in this thread? Is there some silly implied criticism of Hillary for using a cellphone to conduct her business?
If so, that would be beyond absurd. Government officials need to be able to communicate quickly with just about anyone. Get rid of a cellphone, and then she'd have to use a computer, which could be hacked. Get rid of the computer, and have her only use landline telephone calls. Not only would that be inefficient, but those too could be bugged, or if the other person is untrustworthy they could have someone listening in. And so on.
A criticism of an official simply for using modern means of communication, without more, would be absurd. So I hope that's not what was being implied in the OP...