T1 Media Gateway (TIMG) and PBX IP Media Gateway (PIMG) devices are used in order to allow PBXs to use Unity Connection as their voicemail system. The communication from TIMG/PIMG to Unity Connection is via Session Initiation Protocol (SIP). The communication from PBX to Unity Connection is via Time-Division Multiplexing (TDM).
This document describes an issue that might be encountered with this type of integration.
Unity Connection is configured to work with TIMG in order to integrate with PBX. When you call a PBX and Ring No Answer or Call Forward All to voicemail, the call goes to the Opening Greeting instead of to the voicemail greeting.
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TIMG composes a invite because it has received the physical call. However, at this point, TIMG has not received any call information from the PBX.
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The main issue here is that the call information from the PBX arrives about four seconds after the physical call that triggered the Invite to be sent to Unity Connection is received. Thus the call rolled to the Opening Greeting because no Diversion: was contained in the inital invite to Unity Connection to tell Unity Connection to what mailbox to route the call.
In order to fix this issue, navigate to Configuration > TDM > General, find Maximum Call Party Delay (ms), and change it to a value such as 5,000 ms. This adds a five-second delay before the initial invite is composed to Unity Connection, which allows time for all call information to be received from PBX.