@pichlo
Try the following:
delete Voicemail from contacts
go to phone/settings/voicemail-number, select the (only) entry
check that in phone/select contact/voicemail you see correct number
try again

The occurring that only number is displayed is due to you have your voicemail number stored in another contact in your addressbook,.so the app does not know which to show and displays just number (fine to me, as I have saved it also into a contact; remember: voicemail is not a contact).