Problem: Signaling
Multiple signaling systems are generally required - a VoIP signaling protocol such as SIP, H.323, MGCP - and the signaling protocol used to connect VoIP gateways to external voice networks - e.g. ISDN Q.931. Problems can arise due to incompatibilities between VoIP signaling protocol implementations (e.g. phone and gateway) as a number of IP PBX vendors have made proprietary extensions to standard protocols. Problems can also occur between an external and internal signaling system - for example if the gateway that converts from one to the other has been incorrectly configured and is effectively blocking certain features.
Impact
User may be unable to establish a call.
Resolution
Review configuration of call control system and IP Phones
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