Softswitch and IP PBX vendors currently do not have MoH capabilities built into their systems. They require SIP
enabled audio streams because SIP is the only way they can switch any audio to another audio stream (such as the caller audio part of a phone call). There is currently no standard way of providing MoH in the SIP world, but equipment vendors can provide this feature by transcoding MP3 streams or other types of audio streams to Real-Time Transport Protocol (RTP), which is the protocol used for voice transport.
IP modules on traditional PBXs do not suffer from this limitation as they can take advantage of the existing MoH capability within the PBX.
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