In secure areas that deal with classified information personal cell phones are not permitted. There are electronic gadgets which detect cell phones in case one "forgets" that he has it in his pocket.
Hopefully Homeland Security is mostly in a classified area.
Not all of them. It depends on the level of classification, because complete security requires Tempest certification (basically a large Faraday cage). Inside, cell phones simply won't work.
But, there's a very good reason for keeping people out of personal email accounts (and even unverified websites) on any government computer: it's the mostly likely vector for compromise.
Hopefully Homeland Security is mostly in a classified area.
Johnson's office may be, but not everyone else -- at least not in an area that is Tempest certified. It's enormously expensive to house that many people.