This configures whether this card's ISDN D-channel signaling is Associated, where each PRI carries its own signaling, or Non-Facility Associated (NFAS), where one D-channel backs up signaling for multiple PRI spans. Getting this wrong specifically breaks call setup on any span that's supposed to be sharing a backup D-channel under NFAS: set to no-nfas(1) when the actual wiring depends on a shared or backup signaling channel, calls on the spans without their own dedicated D-channel simply won't establish. For example, a group of four T1 PRI spans configured to share one D-channel via NFAS will fail to set up calls on three of the four spans if this object is mistakenly left at no-nfas(1).