I've only tried the bluetooth model in a test-drive car, and I haven't yet got our car (due Aug), so I can't provide you the "offical" answer that probably sits in the users manual, but here's my opinions:

On your device, check if bluetooth is visible for other devices (menu -> settings -> connectivity -> bluetooth -> visibility -> visible for all)

On the bluetooth module there should be some trick to enable more than one device, and to select which one to use. On the test drive, we couldn't also pair our own device until we removed all existing paired devices (from previous test-drivers probably) and started all from the beginning.
Remove the nokia and try to pair the sonyericsson. If it works, then try to re-pair the nokia and check on the manual how to switch between them.

If you need any hint on the SonyEricsson device (K750i and others) just message me.

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