A while ago there were some complains from people on both dev and users mailing lists that the "+" sign wasn't transmitted to the protocol. So, I just wanted to let you know that this is now fixed. For more information see issue 973: http://java.net/jira/browse/JITSI-973.
Additionally we have added the possibility to add prefixes to contact sources like MacOSX Address Book, Ms Outlook, Ldap and Google Contacts. Check the new configuration form in Options/Advanced/Contact sources.