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I think the in exchange it should be configured and allowed in outlook. Check this url it is for Exchange 2010 sp1 but still it applies to exchange 2013 as well.<br />
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<a href="http://www.stevieg.org/2010/06/calendar-sharing-improvements-coming-in-exchange-2010-sp1/" rel="ugc">http://www.stevieg.org/2010/06/calendar-sharing-improvements-coming-in-exchange-2010-sp1/</a>
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The workaround is the one that actually works.
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Shared calendar to user with iPhone. User receives email and able to subscribe to calendar.<br />
<b>Subscribed calendar only shows &quot;busy&quot;.</b><br />
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Changed following settings on permissions:<br />
- Default as &quot;author&quot;<br />
- Subscribed user as &quot;Free/Busy Full Details&quot;
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Shared calendar to user with iPhone. User receives email and able to subscribe to calendar.
Subscribed calendar only shows "busy".

Changed following settings on permissions:
- Default as "author"
- Subscribed user as "Free/Busy Full Details"

Exchange 2013 Shared Calendar on iPhone

Exchange is the server side of a collaborative application product that is part of the Microsoft Server infrastructure. Exchange's major features include email, calendaring, contacts and tasks, support for mobile and web-based access to information, and support for data storage.

Exchange

Microsoft Outlook is a personal information manager from Microsoft, available as a part of the Microsoft Office suite. Although often used mainly as an email application, it also includes a calendar, task manager, contact manager, note-taker, journal, and web browser.

Outlook

The iPhone is a smartphone made by Apple Inc. It and the iPad, along with several other mobile devices from Apple, run on the iOS operating system, which has its own topic, as does the iPad for device-specific questions. Native iOS applications are written in Objective-C or in Swift using the Cocoa Touch frameworks or HTML5 compiled with the help of a package manager such as Adobe PhoneGap; a common IDE is Xcode.