yes, your phone will not work.
i have the same problem with my pocket pc phone, will pair with x5 suceesfuly but you cant transfer the sound to the x5 or recieve call

i think the reason because those phones they dont support the hands free profile in the X5.

So the good news for you you are with cingular thats mean GSM, all you need to buy cheap phone like SONY T68i if you can find, or the V551 from cingular and both they work 100% with x5 incluiding dialing phone book from the steering wheel.

swab the SIM CARD when you are in the X5 in t68i or v551

i hope thats help