Me, I would install my dial plan outlined in my notes, tweaked for your needs. You're not really blocking calls, just routing them as desired.
Set the Primary Line is Trunk Group 1 = sp1,li1. All calls will go out the default trunk sp1 (GV), if available and permissible by that trunk's digit map. Failover will be to trunk li1 (POTS), if available and permissible by that trunk's digit map. You can still route any call to any trunk manually using the **service access codes, if available and permissible by the trunk's digit map. The user will just make calls normally.
Build the dial plan.
Configure the settings.
Test it.
OE
