Hi Lukas,
This is due to a Group Policy on the other side.
In some chats in Microsoft Teams Free, you might not have every feature you’re used to. When chats are created between users with a work or school account (managed by an organization) and users with a personal account, only certain options are available.
Important: Policies that organizations put in place for their members might affect chats between personal and organization members, including data retention, removal of personal users from organization chats, or certain users displaying as Unknown or Anonymous. Organizations must enable chats with personal users for organization members to be searchable.
For a Personal to Organization > Video calls are set to NO.
I hope you find this information useful. If you have any more questions, don't hesitate to reach out. Otherwise, please take a moment to accept the answer. Wishing you good luck!