A well-known instant messenger has been used in the past for the communication. However, the cost was felt to be out of all proportion to the benefit.
There was no direct connection to a calendar and there was no support.
After intensive research and analysis, vivo finally found timee and now handles the entire communication and organization with it.
The decisive reasons for this are the high level of clarity, the low cost and ease of use , DSGVO compliance and local contact persons with fast reaction times.