Apple's iSync supports a lot of phones (and we don't have time now to get into why it doesn't support all phones), but that support for some phones can be a little wonky - namely Sony Ericssons (and I just happen to own a W800i, one of the victims). For these phones, iSync supports syncing just fine, but Address Book might not be able to talk to the phone for SMS sending or phone number dialing. Enter this article at mobile.feisar.com which offers a simple workaround for fixing this broken support and enabling Address Book with the true Sony Ericsson super powers it deserves.
As an added bonus, the article even offers a download link for Apple's Property List Editor, which I believe is a tool that can only be had by installing the Developer tools from a Mac OS X installation DVD. This simple little app can come in handy, however, for all sorts of file tinkering hints like this one, as well as many from the likes of macosxhints.com.
Check out the mobile.feisar hint to get Address Book working with your somewhat-supported Sony Ericsson phone, and just in case you're getting any ideas: I have no idea if this simple tip can work for other brands/phones, so proceed at your own risk.