Hi mate
Installed quite lot of 6800 on A(MiVoice)400 wit various versions of firmware and can agree with you that there is a lots of rooms for improvements...
Don't have problem with dialling or dialling from SW (Mitel dialler???) but with MWI (6867 and 6869 - correction required on cfg file), callback on Line keys,...Functionality - whole call handling procedure (transfer, hold) on low end phones (6863 and 6865)...
Haven't tried 6900 series (not available yet on on MiVoice400) but looking on them, expecting same issue as design is based on 6800 series...
And yes, RIP Aastra
BR