I bought a Motorola C350 GSM phone here in the UK as it's listed on Apple's web site as being iSync-compatible. I hooked it up using a mini-to-normal USB cable I had lying around, but iSync refused to add the phone as a new device, even though I could see the phone listed in OS X's network preferences.
A quick Google suggested that I might need to use Motorola's own cable rather than a generic one - luckily before I spent £20 plus on one, a colleague suggested I look at the iSync preference file for my phone - you can do this from the Terminal and Emacs, or do as I did and log in as root and find it in BBEdit... it's /System/Library/SyncServices/MotorolaConduit.bundle/Contents/Resources/USBDevice.plist and I discovered the Product ID in the plist file for the C350 (14338) didn't match the Product ID listed for my phone in OS X's System Profiler (22530). Once I'd changed this, bingo! iSync could see my phone and happily sync using the cheap USB cable I got with a keyring camera!
So perhaps the Motorola C350 phone has a new product ID or a different one in Europe or the UK?
This is an extract from the file with the changed Product ID:
<key>C350_GSM</key>
<dict>
<key>idProduct</key>
<integer>22530</integer>
<key>idVendor</key>
<integer>8888</integer>
</dict>