Allow guests to join immediately teams sessions
Unlike Skype for business, Teams does allow to configure the behaviour when a guest join the session, nor via PC or directly by phone call. They always get sent to waiting room until moderator joins
John Gillanders commented
You can already configure this in two ways:
1. For all calls via the Global policy in Teams admin center - https://admin.teams.microsoft.com/policies/meetings
2. For an individual call by clicking the 'Meeting settings' link in the meeting invitation and adjusting the setting there.