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