KAS55,
For test 2, change to COS on a desk set to match the COS of the trunk being used for DISA.
Then try test 1 again from the same set (but using trunks COS)
This will prove if the trunks COS is capable of performing the task as opposed to testing via the trunk.
For the Nupoint solution, I don't know how it could be done, I'm only confident it can. Maybe someone else has already set something up similarly and can voice an opinion. Refesher - looking for Nupoint to capture digits dialed by caller and then dial same digits on internal port after dialing prefix digits to set forwarding. Some sort of callback feature for example but used for a different purpose.
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Occam's Razor - All things being equal, the simplest solution is the right one.