I have an early 04 and have done a ton of research on this.

Check your options and look for telephone pre-wiring and/or BMW assist.
If you have either, then at a minimum, you'll need a newer (2005/2006) TCU. They show up on eBay pretty frequently for about $100.

If you have the BMW Assist option, then you'll have the microphone, bluetooth antenna and all the wiring. Your TCU (located in the rear compartment with the stereo amp and CD changer) will need to be replaced. You may also need the voice control connector loopback plug installed. Take a look behind your CD changer and you should find a connector that's got a plug with some looped wires installed. If that's missing you'll need to install one.

Depending on what options are installed, your vehicle might need to be coded too.

Don't be fooled by the Bluetooth logo and pairing code on your existing TCU... it won't pair with your phone.

I have a bunch of docs related to this upgrade. PM me with your email address and I'll send you what I've got.

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'04 E53 X5 4.4i
'05 Mazdaspeed MX-5
'00 Saab 9-3 SE Cabrio