ircuit Forward Backward 1. Idle Tone on Tone on 2. Seized Tone off Tone on 3. Answered Tone off Tone off 4. Clear back Tone off Tone on 5. Release Tone on Tone on or off 6. Blocked Tone on Tone off Source: Ref. 6. a signal sequence that has six characteristic operating conditions. Let us consider several of these conditions. Seized The outgoing exchange (call-originating exchange) removes the tone in the forward direction. If seizure is immediately followed by release, removal of the tone must be maintained for at least 100 msec to ensure that it is recognized at the incoming end. Answered The incoming end removes the tone in the backward direction. When another link of the connection using tone-on-when-idle continuous signaling precedes the outgoing exchange, the "tone-off" condition must be established on the link as soon as it is recognized in this exchange. Clear Back The incoming end restores the tone in the backward direction. When another link of the connection using tone-on-when-idle continuous signaling pre- cedes the outgoing exchange, the "tone-off" condition must be established on this link as soon as it is recognized in this exchange. Clear Forward The outgoing end restores the tone in the forward direction. SIGNALING Table 7.7 Audible Call Progress Tones Commonly Used in North America Frequencies Cadence 350 + 440 Continuous Busy (station) 480 + 620 0.5 sec on, 0.5 sec off Busy (network 480 + 620 0.2 sec on, 0.3 sec off congestion) Ring return 440 + 480 2 sec on, 4 sec off Off-hook alert Multifrequency 1 sec on, 1 sec off Recording warning 0.5 sec on, 15 sec off Call waiting 0.3 sec on, 9.7 sec off Source: Ref. 7. Table 7.8 North American Push-Button Codes Dial Pulse Multifrequency Digit (Breaks) Push-Button Tones 941,1336 Hz 697,1209 Hz 697,1336 Hz 697,1474 Hz 770,1209 Hz 770,1336 Hz 770,1477 Hz 852,1209 Hz 852,1336 Hz 852,1477 Hz Source: Ref. 7 Blocked At the outgoing exchange the circuit stays blocked as long as the tone remains off in the backward direction. 7.3.3 Subscriber Call Progress Tones 