I've got an 'Interphone' (the previous model) which pairs fine with my iPhone and Garmin GPS but I've never been able to get the MacBook Pro to pair with it. You're running Bluetooth Setup Assistant and selecting headset right ?
I know the Z9i is supposed to pair with up to 5 devices at a time (two of them phones) but I'm not 100% sure it will work with the computer in the way you're looking for, their FAQ might help or send them an email (I've always found them very helpful)
http://www.myblueant.com/products/headsets/z9i/z9i_faq.php
