The best was to do this if you want distinctive ringing on handsets dependent on ring group it to set the “Alert Info” in each ring group. See here for what to set this to on Aastra handsets. I’ve used this (quite some time ago I have to admit) on some old Aastra handsets: https://wiki.freepbx.org/display/FOP/Aastra (see the “Alert Info” section.
It’s much easier on Sangoma handsets (as Tony said) as these can be simply selected from a pull down menu for each ring group.