Re: How to separate the preferences for each deskphone registered to the same object (when "Store Phone Config" + "Discard Config on Phone" is used)?
A solution for your problem could be prohobiting changes on the phone. The German user Muschelpuster made a very helpful Excel sheet for doing just that:
http://forum.innovaphone.com/moodle2/mod/forum/discuss.php?d=21324
This should help.