Many rank the Parrot Minikit Bluetooth as the best of these devices. I've had one for around six months and am very happy. It's easy to use, connects reliably to my mobile, loud and clear, and has excellent battery life. There's no number display though. However, if you've recorded a name for your contact, then it will say that name before ringing.
It also comes supplied with a mains charger and car charger. You can charge it while using it which, unbelievably, not all of them allow you to do.
There's a strong clip which keeps it in place on your sun-visor and it's obviously portable from vehicle to vehicle as there's no wires.
Steal for 50 quid really.
P.S. The Parrot has an outstanding noise-cancelling microphone, so voice quality for the person at the other end is top-class. How it does it I don't know - but everyone says it's like I'm calling from home !