The changes from this post about the S700i also work for the P990i (the string is P900).

To collect it in one place - this is taken from http://www.macosxhints.com/article.php?story=20050731124746116

Quit and backup your Address Book app, etc.
Control click on the Address Book app, select Show Package Contents, then navigate to Contents -> Resources.
Control click on Telephony.bundle, select Show Package Contents, then navigate to Contents -> Resources
Open ABDeviceCommandSets.plist with Property List Editor (or a text editor to change the XML in a more primitive way)
Find this array in the ABDeviceCommandSets.plist: Root -> 2 -> ABDeviceModelStrings ... you should see one string entry here: K700. Add another string entry with the value P990.
Save and quit

Note - if you are running with a non-admin account (and if not then why not?) then this will need to be done as an administrator.