The Alert-Info Interoperability Problem (1)
The RFC 3261 allows an UAS or proxy to provide  a specific
ring- /ringback-tone as a reference (e.g. HTTP URI) which can
be played to the user in the Alert-Info header.
This mechanism does not ensure interoperability when there
is no common understanding of the referenced content
(different countries or vendors, hearing impaired) or when the
user uses his own tones configured in the end device.
For interoperability for services as Call Waiting, a mechanism
is needed which allows the callee‘s UA to transmit to the
caller a semantic indication which signals that the call is a
waiting call and allows the caller’s UA to decide how to render
the received information to the user.
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