We're looking at putting sipXecs into a customer that already has a bunch of Aastra 5x series phones. Problem is that Aastra managed phone support isn't coming to sipXecs until version 4.0.
However, it is possible to take the plugins from the development version of sipXecs (currently 3.11.
Built a 3.11.9 box and grabbed aastra-phones.jar from the \usr\share\java\sipXecs\sipXconfig folder.
Copied aastra-phones.jar to \usr\share\sipxpbx\lib folder on 3.10.3 box.
Copied all files from http://sipxecs.sipfoundry.org/ViewVC/sipXecs/main/sipXconfig/plugins/aastra/etc/ to /etc/sipxpbx/aastra.
Changed owner of files and folder to sipxchange ( 'chown sipxchange:sipxchange /etc/sipxpbx/aastra' and 'chown sipxchange:sipxchange /etc/sipxpbx/aastra/*' )
Restarted sipxpbx services (service sipxpbx restart).