How do I get one-touch join working reliably on Microsoft Teams Rooms?

One-touch join on Microsoft Teams Rooms almost always fails for one of five reasons, and they're all configuration rather than hardware: the room resource mailbox is misconfigured in Exchange, the meeting was forwarded into the room rather than originally inviting it, ProcessExternalMeetingMessages is disabled, the organiser's invite came from a non-Teams calendar source, or the MTR account has stale credentials. Work the list in that order.

The five failure patterns and what to check:

1. Resource mailbox misconfigured. The room must be a resource mailbox in Exchange Online, not a regular user. Calendar processing must be enabled, with AutomateProcessing set to AutoAccept, AddOrganizerToSubject enabled, DeleteComments disabled, and RemovePrivateProperty disabled. Without these the Teams meeting body never makes it onto the panel and the Join button never appears.
2. Meeting forwarded rather than invited. If a user books a meeting on their own calendar, then forwards the invite to the room afterwards, the room's calendar entry has no embedded Teams join URL. The fix is process: book through Outlook's room finder or the Teams scheduler so the room is a true attendee from the start, not a forwarded recipient.
3. ProcessExternalMeetingMessages disabled. For meetings organised outside the tenant (a partner, a client, an external Teams meeting) the room must be allowed to process external meeting metadata. Run Set-CalendarProcessing -ProcessExternalMeetingMessages $true on the room mailbox. This is the single most common cause of "the panel is blank for external meetings only".
4. Non-Teams calendar source. If the organiser is on a third-party calendar (Google, on-premises Exchange not federated to M365), the meeting URL may not register in a format MTR recognises. Direct Teams Calendar (Coordinated Meetings) or a properly federated hybrid Exchange is the durable fix.
5. Stale MTR account. The room signs in to a dedicated M365 service account. Password expiry, MFA challenges (which cannot be answered on the panel) or licence lapses kill calendar sync silently. The fix is operational: exempt MTR service accounts from interactive MFA, set passwords to non-expiring with conditional access protection, and monitor licence assignment.

If one-touch join is failing across multiple rooms simultaneously, look at tenant-level changes (calendar processing policy update, Conditional Access rule, licence reshuffle). If it's failing on one room only, the resource mailbox or that room's account is the issue. The Microsoft Teams Rooms commissioning checklist should cover all five at deployment, and ongoing support catches the operational ones (account, licence, policy drift) before they hit users.

Quick reference: check resource mailbox calendar processing first, then meeting origin (forwarded vs invited), then ProcessExternalMeetingMessages, then calendar source, then service account hygiene.