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4.4 MFC-R2 SIGNALING 103TABLE 4.4-5International R2 Backward Interregister SignalsGroup AA-1 Send next digit (n þ 1)A-2 Resend digit (n 2 1)A-3 Address complete, prepare to receive a group B signalA-4 Congestion in national networkA-5 Send calling patty categoryA-6 Address complete, end of register signalingA-7 Resend digit (n 2 2)A-8 Resend digit (n 2 3)A-11 Sent country code indicator (transit seizure)A-12 Send Z-digit (terminal seizure)A-13 Send nature of circuitA-14 Send echo suppressor informationA-15 Congestion at International Switching Center, or all trunks busyA-9 and A-10 are not usedGroup BB-2 Send special information toneB-3 Subscriber line busyB-4 CongestionB-5 Unallocated numberB-6 Subscriber idle, chargeB-7 Subscriber line idle, do not chargeB-8 Subscriber line out of serviceB-9 through B-15 are not usedSource: Rec. Q. 441. Courtesy of ITU-T.Signals 0-11, 0-12, and 0-14 indicate a transit seizure. These signals imply thatthe called number includes a country code and are also known as country code indicators.In addition, the signals contain information for the control of echosuppressors.Group I Signals. The forward signals I-1 through I-10 represent the digits of thecalled number. I-11 and I-12 are addresses for groups of international operators, andI-13 is the first digit of addresses that specify a particular type of test equipment atthe ISC.Group II Signals. These forward signals indicate the calling party category andare sent to exchanges in the destination country when end-to-end signaling is possible.In practice, only II-7 is used.The backward signals are listed in Table 4.4-5.Group A Signals. Signal A-1 requests the next address digit. Signals A-2, A-7,A-8, A-11, and A-12 are sent, just before cut-through, by a transit ISC that has