The compatibility wheel is located on the back of the base.
It is lettered A -G, although it is really hard to see! Most people will say there are no letters there at all. Just make sure that the dial is up as far as it goes or down as far as it goes.
Most phones work best on A or G (all the way up or all the way down).
A card is included with the 9120 with compat settings for common phones also for easy reference!