Sounds like your Bluetooth Connection is not strong enough, in My X5 I have to place the phone inside the rest arm compartment or extremly close to it to get a solid connection where I can make and receive calls (this is awkward if you have a large mobile as the holder was designed for the smaller mobiles of the period... Mine is a 2004 X5 and I use my N95 telephone all the time...

My phone barely squeezes into the compartment with the holder removed, I normally half close the cover for the drinks holder just below the armrest and place it on top for a solid connection...

I also notices that sometimes it takes a while to connect fully, so give it a few minutes before attempting to make a call...
If the mobile is not close enough you might appear to receive calls and have access to your phone book but won't have enough juice to actually communicate with anyone...