Use Of Secure And Non-Secure Telephones
(From the NSA Handbook)
Two separate telephone systems have been installed in NSA facilities for use in the conduct of official Agency business: the secure telephone system (gray telephone) and the outside, non-secure telephone system (black telephone). All NSA personnel must ensure that use of either telephone system does not jeopardize the security of classified information.
The secure telephone system is authorized for discussion of classified information. Personnel receiving calls on the secure telephone may assume that the caller is authorized to use the system. However, you must ensure that the caller has a "need-to-know" the information you will be discussing.
The outside telephone system is only authorized for unclassified official Agency business calls. The discussion of classified information is not permitted on this system. Do not attempt to use "double-talk" in order to discuss classified information over the non-secure telephone system.
In order to guard against the inadvertent transmission of classified information over a non-secure telephone, and individual using the black telephone in an area where classified activities are being conducted must caution other personnel in the area that the non-secure telephone is in use. Likewise, you should avoid using the non-secure telephone in the vicinity of a secure telephone which is also in use. | 
