“impersonating phone company employees “
No law against this.
Impersonation of anyone to gain access is at a minimum trespass. And federal offices and buildings enforce that. Senate offices are considered federal property even if they aren’t. There is also a security issue when discussing members of Congress.
It may also fall under the terrorist statues.
The charging FBI affidavit says trespass ... It was a really bad decision to do whatever they were doing at a federal office.