Cheers, thanks for that advice. First time I could proceed with a "premium" call
Ok, two more things (both can be found at the log)
1) After leaving the car I have the impression it takes ages before the phone registers at the network again. Sometimes I have to "play" with the phone a bit.
2) Today it connected not as "premium", for whatever reasons. (edit: I did some more testing, the second time it always connects as "standard" - I have to deactivated Bluetooth and switch it on again).
p.s. the phonebook app works lovely.