Horde and its SyncML package are Open Source projects. We neither have the time nor the resources to test our SyncML implementation against all the hundreds of phones and clients out there. So we rely on you (yes you!) to provide testing data for individual clients.
This is done by creating a Test Case. A test case for SyncML is basically a recording of a successful sync session. The testsync.php script can replay such a recording by sending the recorded client requests to the SyncML package and check if the response matches the one in the original recording. These test cases allow us to verify that changes or new features don't break things for existing phones. So if your phone is not listed yet in the list of test cases below: create one ore more test cases as described in SyncMLCreateTestCase - you'll enjoy a much more stable SyncML replication for your phone!