ETSI ETS 300 057 ed.1 (1992-05) - Integrated Services Digital Network (ISDN); Call Waiting (CW) supplementary service; Functional capabilities and information flows
ETSI EN 300 392-10-11 V1.3.1 (2004-01) - Terrestrial Trunked Radio (TETRA); Voice plus Data (V+D); Part 10: Supplementary services stage 1; Sub-part 11: Call Waiting (CW)
RM-31 Morse key - 1960 (eng) - Czech army green radios