GigaSet C610a IP and MWI

Have freepbx running with a GigaSet C610a IP connected to it. As I have multiple accounts I have setup all 6 lines on the GigaSet so I can see what outside number was called (named extensions with the outside number).
Also for most of the lines I have voicemail enabled. I have found a post somewhere online how to link all extensions to the main extension (601) voicemailbox. So in case somebody leaves a voicemail there is no need to listen to all separate boxes.

Somehow without configuring the display of the Siemens is able to show there is a message left by showing a voicemail symbol in the display. So far so good. This morning I received a call by a scammer (800-Notes) that left a 1 second message. The call was made to the main account that leads to extension 601.
Problem that I now ran into is that the Siemens was showing that there were 6 voicemail messages. Needles to say that when I removed the message (1 new) in mailbox 601 the MWI disappeared.

Any clues?