Teams mobile’s calendar deadline is really an Exchange Web Services problem

On the surface, this is a routine app update: get everyone onto the latest Teams mobile version by October, or they lose calendar access on their phone. Underneath it, this is one small, visible symptom of a much bigger deadline that a lot of IT teams haven’t fully reckoned with yet: Exchange Web Services, the API that’s quietly underpinned Exchange integrations for over a decade, is being switched off.

Rollout begins and completes worldwide, including GCC, GCC High and DoD environments, in early October 2026, pulled forward from an originally later October estimate. Teams mobile users on iOS or Android who aren’t running the latest app version will lose access to Calendar functionality within Teams mobile specifically; desktop and web clients are unaffected.

The minimum versions are iOS 8.9.0 (build 8.9.77.2026092302) or higher, and Android 1416/1.0.0.2026122504 or higher.

The Teams mobile calendar issue is real and worth acting on, but it’s the smallest part of a much larger story. Checking Microsoft’s own EWS deprecation documentation makes the scale of this clear:

• EWS deprecation isn’t new, but the urgency changed sharply in 2024. Microsoft first announced EWS would stop receiving functionality updates back in 2018, with a 2023 announcement setting October 2026 as the disablement date. The January 2024 Midnight Blizzard security incident, a nation-state attack that involved EWS, elevated the priority significantly and widened the scope from third-party applications to every Microsoft product still using it, including Outlook, Office, Teams and Dynamics 365.
• This affects far more than calendars. Teams mobile’s calendar feature is one visible, user-facing casualty. The underlying retirement affects any internal tool, script, add-in or third-party application your organisation has built against EWS, most of which nobody outside your IT team will notice breaking until it does.
• Microsoft has published tools specifically because this is a bigger job than most organisations expect. An EWS Usage Report and an EWS Analyzer tool exist precisely to help organisations find out what’s actually still using EWS internally, because for most IT teams, the honest answer right now is “we don’t fully know.”

• All users accessing Microsoft Teams from iOS or Android mobile devices
• IT teams managing mobile application management or automatic update policies for Teams
• Anyone in the organisation responsible for internal tools, scripts, or third-party integrations that connect to Exchange Online, whether or not they realise those tools use EWS

This is one of the few updates in this set that genuinely requires action before a fixed date, so it’s worth treating with more urgency than a standard heads-up:

1. Prioritise internal and third-party applications for migration now, using Microsoft’s EWS-to-Graph API mapping and analyzer tooling, rather than waiting for parity gaps to close on their own.
2. Confirm Teams mobile app versions across your organisation now, and enable automatic updates where devices are managed, well ahead of early October 2026.
3. Run Microsoft’s EWS Usage Report for your tenant, rather than assuming Teams mobile is the only thing affected. This is the fastest way to find out what else in your organisation is quietly relying on EWS.
4. Treat October 2026 and April 2027 as two separate deadlines, not one. October is when EWS starts being disabled globally; April 2027 is when it’s gone entirely, even for organisations that used the extension policy.

Learn more

• Deprecation of Exchange Web Services in Exchange Online — Microsoft Learn
• Exchange Online EWS, Your Time is Almost Up — Microsoft Tech Community

Based on Microsoft 365 Roadmap update MC1440980, last updated 31 July 2026. Rollout dates are provided by Microsoft and may change.