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100 CHANNEL-ASSOCIATED INTEREXCHANGE SIGNALINGFigure 4.4-5. End-to-end interregister signaling.with A-1. The second through fifth digits are acknowledged in the same way, and thesixth (final) digit is acknowledged with A-3. Exchange X then sends the calling partycategory (II-2), and exchange Z ends register signaling with a group B signal.R2 end-to-end signaling in national networks is also possible on connections thatpass through several intermediate exchanges.4.4.5 R2 Supported Features in National NetworksWith end-to-end R2 interregister signaling, information about the status and natureof the called party can be sent from the terminating to the originating local exchange.In addition, the calling party category (subscriber, operator, etc.) is sent from theoriginating to the terminating exchange. The combination of these procedures supportsa number of national network features that cannot be provided by Bell SystemMF signaling. Some of these features are outlined below, using the call of Fig. 4.4-5as an example.Free Calls. Certain lines on a local exchange may be marked as “free” destinations.During the register signaling on a call to a free destination at exchangeZ, this exchange can indicate to originating exchange X that the call should notbe charged (signal B-5).