did you ever figure this one out, and I’m trying to do the same thing, and having a tough time understanding the directions on how to get sla to work.
Running 3 mitel 5220’s, one google trunk auto configured through the piaf gv module,
No fxo/pstn or any other conectivity
so far I have blf working but just can’t figure out the sla side. and yes the phones support Shared line just the code seems confusing to me especially when trying to figure out how to trigger subscribe to the google line… if that makes any sense