Terminal Devices Used in Connection with the Public Telephone Service (Other Than Telephones) - Telephone Network and ISDN Operation Numbering Routing and Mobile Service (Study Group 1) 7 pp
Many terminal devices are available that may be substituted for, or used in conjunction with, subscribers' telephones. This Recommendation outlines some difficulties that may arise from this practice, particularly for the calling party. It also offers some guidelines to be considered when setting national requirements for terminal equipment. This Recommendation does not address issues related to PBXs, and it may not be applicable for telephone services entirely provided via the ISDN.