| 摘要 |
Aspects relate to call control by third parties, where IP or a similar transport is used over a part of the channel for a given call and circuit-switched signaling protocols are used on a different part. For example, a PBX can communicate using SIP signaling to a gateway to a PSTN network, to which a Public Land Mobile Network (PLMN) is coupled, and which a mobile device, uses for voice communication. Controlling or updating status in these kinds of situations can be accomplished by forming a Q.931/932 message, such as a FACILITY message, which has fields (Information Elements) for pre-defined purposes (other than third party call control), and providing control or status information in such field(s). The message (the info) is tunneled through the gateway and the network(s), and arrives at its destination device. That device determines that a field does not comport with a valid format interpretation, and on that basis, can determine that the field has contents to be interpreted as call control or status information.
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