Voicemail support between Bintec be.ip plus and T48G

I see following behavior in a small office I support in IT matters. We are using a Bintec be.ip plus as telephone center and T48G phones (recommended by me, so I have the fun to find the solution).

Calls are working well, voice mails are problematic.
The users can playback (all) voice mails through the hotkeys and delete them one by one while hearing.
But neither the MWI light is signalizing a waiting message, nor the display. There is also no message shown in the list, which is unfortunate, if the people wish to follow up a call or delete outdated messages without hearing all of them again.

Last tested with firmware 35.80.0.95 on phone and latest firmware of Bintec device.

Related settings done on the phone:
Account/Advanced
Subscribe for MWI: Enabled
Subscribe MWI To Voice Mail: Enabled
Voice Mail: *50
Voice Mail Display: Enabled

Features
Display Voice Mail Popup: Enabled

For what I know I also performed all necessary settings on the Bintec telephony device, but with no luck.

Best greetings from Germany
Olaf