If the calendar tab in Teams on your phone went blank this week, you are probably not looking at a bug. Microsoft started switching off Exchange Web Services, the nearly 20-year-old interface that lots of apps use to read Outlook mailboxes and calendars, on October 1, and the Teams mobile calendar is one of the first places regular people will notice it. The change runs tenant by tenant, so some offices will see nothing yet while others are already fielding help-desk tickets.
This is the explainer we wanted when we read Microsoft's notices: what actually flipped on October 1, why an app update fixes the Teams problem, and what an IT admin still has time to do before the hard stop on April 1, 2027.
What changed on October 1
Exchange Web Services, usually shortened to EWS, is the old SOAP-based doorway into Exchange Online. Microsoft stopped adding features to it in 2018 and has been steering developers to Microsoft Graph ever since. In February the Exchange team laid out the endgame in a post titled "Exchange Online EWS, Your Time is Almost Up", and the mechanics come down to one tenant setting called EWSEnabled, which can be True, False, or blank (Null). Here is how each value behaves, taken from that post:
| EWSEnabled | Before October 2026 | From October 2026 |
|---|---|---|
| Null (the default) | All EWS allowed | Flipped to False during rollout, which blocks all EWS |
| True | All EWS allowed unless an allow list exists | Only apps on the AppID allow list get through |
| False | All EWS blocked | All EWS blocked |
The important row is the first one. Most organizations never touched this setting, so they are sitting on Null, and Microsoft is now changing Null to False as the rollout reaches each tenant. That blocks every app still using EWS at once. An admin can set it back to True afterward, but Microsoft warns there will be a service interruption in the meantime, so anyone who did nothing in September is going to find out which apps depend on EWS the hard way.
There is a safety net, with a catch. In a September 4 follow-up, Microsoft said it will pre-populate an AppID allow list for tenants that never built one, based on the previous 60 days of EWS usage. That sounds generous until you think about which tools only run now and then. A quarterly export job, a year-end archive script, or a migration tool someone used in the spring would not show up in a 60-day window, so it may not make the automatic list. Microsoft says so itself: the auto list "may miss applications that run infrequently, and it may include apps you no longer want to have access."
Why Teams on your phone needs an update
The Teams piece arrived separately in Message Center post MC1440980, published July 27 and revised on July 31. It says people using Teams on iOS or Android must run the latest app to keep Calendar working, and that older versions will lose calendar access. The deadline moved from late October to early October in that July revision, which lines up with the EWS schedule. Microsoft never says in so many words that EWS is the reason, but the notice tells admins they can extend EWS until April 2027, and the German outlet heise online drew the same connection. Our read is that older Teams mobile builds still fetch calendar data the old way, and the new builds don't.
These are the minimum versions Microsoft lists:
| Platform | Minimum Teams version for Calendar | Affected? |
|---|---|---|
| iPhone and iPad | 8.9.0 (build 8.9.77.2026092302) or later | Yes |
| Android | 1416/1.0.0.2026122504 or later | Yes |
| Windows, Mac, and web | No change | No |
So if you are a regular Teams user, the fix is boring and quick. Update Teams from the App Store or Google Play, and turn on automatic app updates while you are there. If your phone is managed by work and the update isn't showing up, that is a question for your IT team, because Microsoft specifically asks admins to check that their mobile management tools are pushing the latest build.
How we got here, and how long is left
This has been a very slow goodbye. Microsoft's own deprecation page gives the short version:
- July 2018: EWS stops getting new features.
- 2023: Microsoft sets October 2026 as the date EWS gets disabled in Exchange Online.
- January 2024: the Midnight Blizzard attack, which involved EWS, raises the urgency and widens the cleanup to Microsoft's own apps, including Outlook, Office, Teams, and Dynamics 365.
- February 2026: Microsoft announces the phased, admin-controllable shutdown.
- October 1, 2026: tenants still on Null start getting flipped to False.
- April 1, 2027: EWS is off for good, and admins lose the ability to change the setting.
Counting from today, October 5, that leaves 178 days until April 1 (26 days left in October, then 30, 31, 31, 28, and 31 for November through March, plus April 1 itself). That is about six months, and Microsoft's FAQ is blunt that "there will be no exceptions past April 2027."
The bigger worry for some companies is that not everything has a replacement yet. Microsoft's roadmap still lists gaps targeted for the fourth quarter of 2026, such as full-fidelity import and export for archive and public folder mailboxes and a way to report a message as junk or phishing through Graph. It also names three things that will never come to Graph: generic public folder item access, generic Microsoft 365 Group mailbox access, and access to legacy Discovery Mailboxes. If a line-of-business tool leans on any of those, it needs a different design, not just a code port.
Our take: who should do what this week
If you just use Teams at work, update the app and move on. That is genuinely all most people need to do, and it takes less time than complaining about the missing calendar in a group chat.
If you run Microsoft 365 for a company, this is worth an hour today rather than next month. Check whether your tenant was flipped to False, look at what is on your AppID allow list (whether you built it or Microsoft did), and compare it against the EWS usage reports in the admin center with the infrequent jobs in mind. The 60-day window is the part we would worry about most, because the apps it misses are exactly the ones nobody remembers until month-end or quarter-end. Then treat April 1 as a real deadline and push any vendor still on EWS for a Graph date in writing.
And if you build software that talks to Outlook mailboxes, the clock is now visible to your customers. Every admin who gets burned this month by a surprise block is going to ask their vendors the same question, so it is better to have the answer ready.