I have run into this a few times. Historically when we install CUCM with physical IP Phones no one has noticed because the physical phones have a pretty good amp in them and the user can just adjust the volume up. Jabber/Windows (unclear who's at fault) doesn't provide as much amplification. The only other way to solve this is to adjust the volume on your PRI with intput gain and output attenuation commands.
The dangerous part here is that you can introduce echo into the environment if you allow the returned echo volume to be too close to the originally transmitted audio. You have to be careful to monitor the ERL after making any changes to ensure the echo canceller has enough of a delta to strip the echo from the received audio.
If that document doesn't make sense the only other good place I have seen this documented is in a Cisco press book Troubleshooting IP Telephony (ISBN 1-58705-075-7) starting on page 422).
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