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The default time frame is 15 seconds for Max PDD when the value field is left empty.
B-Timer acts as an extension of PDD and is designed to work as a smaller subset timer to roll over to the next trunk if no response is received from the vendor. The B-timer is activated when the INVITE is sent to the outgoing trunk, and if no 100 Trying is received within the allocated time it will expire and roll over to the next Trunk or Trunk Group if available. B-timer values can be between 1-10 and must always be smaller than the PDD value. The default value entry field for B-timer is empty, which means it will not be considered unless an integer value between 1 to 10 is set for it.
Note:
PDD = (sum of all PDDs from CDRs) / (sum of all completions)where PDD is the defined above