The CallFinder is FXS on a line port of our phone system. Voice volume is low on both sides, regardless of which side originates the call.
Try AT>SPK=1 to increase volume heard on the PBX side and AT>MIC=1 to increase volume heard on the cell side. If volume is still low on either, or both sides, repeat the appropriate command increasing the value to 2. Repeat this until you get volume at a comfortable level on both sides. If these gains get too high, there is the potential of things like echo being introduced into the call, so be aware of this when making the changes.