It's the ULF (phone module) not the I-drive that connects the phone to the car, if it were me I'd swap the ULF from the m3 to the x5 (as a test) and see if the problem goes with the ULF, then newer one (x5) may be a different version (check the part numbers) than the older one (m3) and it may have some differences in the phonebook loading algorithm.

If it does then that's the problem, the newer ULF, (probably tweaked for iphone).

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Sept 03, M3, Silver Grey. Retro-fits: Modded Mk4 DVD Drive, OEM Bluetooth, VR Control, Rainbow Speaker Upgrade, Sony SOT Amp, HK Sub, CCFL, Intravee II, NightMode.