Extended Phone Services (or Device Specific Phone Services) include all service provider defined extensions to the API. The API defines a mechanism that enables service provider vendors to extend the Telephony API using device specific extensions. The API only defines the extension mechanism, and by doing so provides access to device specific extensions, but the API does not define their behavior. Behavior is completely defined by the service provider.
The Telephony API consists of scalar and bit flag data constant definitions, data structures, functions, and callback messages. Procedures are defined that enable a vendor to extend most of these as follows:
For extensible scalar data constants, a service provider vendor may define new values in a specified range. As most data constants are DWORDs, typically the range 0x00000000 through 0x7FFFFFFF is reserved for common future extensions, while 0x80000000 through 0xFFFFFFFF are available for vendor specific extensions. The assumption is that a vendor would define values that are natural extensions of the data types defined by the API.
For extensible bit flag data constants, a service provider vendor may define new values for specified bits. As most bit flag constants are DWORDs, typically a specific number of the lower bits are reserved for common extensions while the remaining upper bits are available for vendor specific extensions. Common bit flags are assigned from bit zero up; vendor specific extensions should be assigned from bit 31 down. This provides maximum flexibility in assigning bit positions to common extensions versus vendor specific extensions. A vendor is expected to define new values that are natural extensions of the bit flags defined by the API.
Extensible data structures have a variable sized field that is reserved for device specific use. Being variable sized, the service provider decides the amount of information and the interpretation. A vendor that defines a device specific field is expected to make these natural extensions of the original data structure defined by the API.
The phoneDevSpecific operation and associated PHONE_DEVSPECIFIC message enable an application to access device-specific phone features that are unavailable via the common Telephony Services on phones. The parameter profile of the phoneDevSpecific function is generic in that little interpretation of the parameters is made by the API. The interpretation of the parameters is defined by the service provider and must be understood by an application that uses them. The parameters are simply passed through by the Telephony DLL form the app to the service provider. An application that relies on device specific extensions will not generally work with other service providers, however, applications written to the common telephony phone services will work with the extended service provider.
phoneDevSpecific
Device specific escape function to allow vendor dependent extensions.
PHONE_DEVSPECIFIC
Device specific messages sent to the application's callback.