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As you noted the type of phone matters as does the type of phone system they use.  If I recall I don't think many of the phone systems will work well with add ins of any kind.  I had an in-line volume control for my last job- gizmo would go between the phone and handset- gizmo plugs into phone base, phone handset plugs into gizmo.  I can't remember how other systems worked in my previous jobs though.  At least one I know I was able to have a volume control handset directly into the phone base (and a flashing light lol- looked like a tiny red beehive)