Speeding up the Establishment and Clearing of Phototelegraph Calls - Telephone Network and ISDN - Operation, Numbering, Routing and Mobile Service (Study Group II) 1 pp
The calling frequency f2 used for phototelegraphy should be different from that used for
telephone signalling J. In the case of automatic or semiautomatic telephone circuits, frequency 500/20 Hz will be adopted as the signalling f2 frequency for phototel