As stated in AR 190-5, vehicle operators on a Department of Defense installation and operators of Government owned vehicles will not use cell phones unless the vehicle is safely parked or unless they are using a hands-free device. The wearing of any other portable headphones, earphones, or other listening devices (except for hands-free cellular phones) while operating a motor vehicle is prohibited. Use of those devices impairs driving and masks or prevents recognition of emergency signals, alarms, announcements, the approach of vehicles, and human speech.  A copy of the Fort Sam Houston policy can be found here.

If your are in the Military, a Federal Employee, or even a Contractor working for the US Government., you need to read the Executive Order from the President of the United States.

Bottom Line: Don't text and drive while on duty and/or in a Government vehicle (including a rental car provided for by the US Government) or even your POV if performing duties for the US Government. Read the Executive Order for more details.