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# CallKit Constants

## Topics

### Constants

[`let CXErrorDomainNotificationServiceExtension: String`](/documentation/CallKit/CXErrorDomainNotificationServiceExtension)

[`#define CX_EXTERN`](/documentation/CallKit/CX_EXTERN)

[`#define CX_CLASS_AVAILABLE(...)`](/documentation/CallKit/CX_CLASS_AVAILABLE)


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