If your phone does not stay in the cradle, it may be because the plastic tab in the handset rest is not flipped the appropriate way.
To fix this, first take the phone off the cradle and look for the small plastic tab pictured below.
You will need to pop the tab up using something flat. A coin or a flathead screwdriver are some good options.
Once it is popped out, rotate it so that the nub is facing upwards, then reinsert the tab into the handset rest. Your phone will now hook onto the handset rest which will prevent it from falling out as easily.
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