Secure calls from this phone could be carried, unencrypted, in shielded cable in ferrous conduit within a secure facility, and would be encrypted via another shared device if the call left the secure facility.
Branded "ELECTROSPACE" on the front. On the bottom is a label, "TELEPHONE ASSEMBLY | 1ST | ELECTROSPACE SYSTEMS INC | SERNO: 6201 PN:89-08103-G07 | REV V U.S. FSCM: 33875". Another small label appears to have been removed adjacent to it. There is a small label/seal, marked "RaytheonE-Systems", which has been punctured to access one of the assembly screws.
This type of phone was used by President Bush on 9/11.
Also known as a "DRSN RED" phone, or "Red Phone". The Defense Red Switch Network (DRSN) is a dedicated, secure. U.S. defense command and control network.
The four red buttons, "FO", "F", "I", and "P", were for the multilevel precedence and preemption (MLPP) system, which allowed the priority for calls to be set ("Flash Override", "Flash", "Immediate", and "Priority", respectively), above the default of "Routine." "Flash Override" allows the President of the United States to ensure his call will be allowed to go through over all others.
The IST used by President GW Bush on 9/11 at Offutt AFB is on display at the GW Bush Presidential Library in Dallas, TX. The only visible difference is the addition of a red sticker at the bottom, notifying the user that calls may be monitored.
A well-known photograph of President Bush also shows him using an IST handset at Barksdale AFB, on 9/11. I have not been able to yet find a photo of the entire telephone.
The IST was replaced by the IST-2 beginning circa 2003.