If you have a phone like the Z520i that is not supported by iSync, you can usually pair it with your Mac, but you can't enable Address Book syncing. But you may be able to get it working if you pair the phone with your Mac from the phone, and not from the Bluetooth app in OS X.
I have tried this with two unsupported Sony Ericsson phones on two different Macs running 10.3.x. You'll find it under Connectivity: Bluetooth: My Devices: New Device on your Sony Ericsson phone.
Mac OS X Hints
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