This sets the overall time budget, starting from the first forward register signal received, within which the entire register-signaling exchange for an incoming call must complete. This is a call-level ceiling rather than a per-digit timer, so it catches scenarios where individual digit timers each pass fine but the cumulative digit-collection process for the whole call is simply taking too long, a distinct failure mode from any single-digit timeout. If register signaling for an incoming call is still incomplete 15 seconds after the first forward signal arrived, with this timer set to 15000 ms, the gateway would abort the call setup even if no individual digit-level timer had yet expired.