You can use the 8 Pool extension list (*0008) as an easy way to do debugging and stopgap routing on NBX systems. The 8 Pool is accessed by prefixing outgoing numbers with 8 instead of 9, and you can manage it from Dial Plan > Extension Lists in the NetSet.
If you are having call audio issues with a particular digital line span, you can remove that span from a regular line extension list, and then add it to your 8 Pool. This will prevent the span from being used for outgoing calls, unless they are prefixed with an 8. You can then modify any settings related to that board or span, and it will not affect any other calls on the system, and allow you to make test calls on only that circuit. Note that this will not prevent incoming calls from avoiding that circuit (you will need remove those extensions from your incoming pretranslator list first).
If you have multiple phone providers, and for whatever reason, you need to route a certain number over a certain provider, you can add only a few channels from that provider to *0008, and then update Table 2 in the dialplan to route that number to the 8 Pool only. In this case, even dialing a 9 to get an outside line would still route over the 8 pool:
TableEntry Create 1 1 5551212 7 7 Local 0 1
DestinationRoute Create 1 Alt Provider for 5551212
DestinationRouteEntry Create 1 1 *0008