Welcome! You can set your ringers by going to Profiles , scrolling to "Advanced" (click) then click the BB button (next to the trackball) and select "Edit", then you can adjust ringers/volume for all events...
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If by assign a number to a phone number you want to assign a speed dial, youcan do that by pressing and holding any letter letter from the home screen. It will then ask if you want to assign a speed dial to that letter.