Translated from the Italian blog:
https://www.3cx.it/blog/docs/timer-sip/
(I didn't find it in the English blog)
What are SIP timers
Blog Author Posted on April 9th, 2019 by Matteo Sala, Training and Technical support for VoIP & UC
SIP timers and VoIP call drop One of the most common error situations in the field of VoIP SIP telephony concerns the sudden drop in calls at 32 seconds.
This is related, in more than 95% of cases, to problems or errors in the configuration of NAT.
To understand why this type of error occurs, it is necessary to introduce the topic of SIP Timers.
The SIP Timers and the drop of calls to 32 seconds. The SIP Timers T1, B, and F are used to primarily determine the time it takes for the remote device to respond before the sender considers it a timeout. That of 32 seconds is a fairly common call drop interval and is essentially dictated by the action of the so-called “SIP timer”. Here's what happens in detail: The Types of SIP Timers: T1, B and F The T1 timer is the estimated round trip time of an IP packet, and the default is 500ms for almost all SIP systems. Timer B is the maximum time the sender will wait to receive an INVITE. Corresponds to a value of 64 times the value of T1. Timer F is the maximum time the sender will wait for non-INVITE messages. Corresponds to a value of 64 times the value of T1. B and F are doubled with each iteration and so an unanswered INVITE will look like this: T0 - sending of the original INVITE. 500ms - sending of the 2nd INVITE. 1000 ms - sending of the 3rd INVITE. 2000 ms - send of the 4th INVITE. 4000 ms - send of the 5th INVITE. 8000ms - send the 6th INVITE. 16s - send 7th INVITE. 32s - send the 8th and the last INVITE. The call is interrupted. Here is therefore explained the reason for the failure of a 32sec call. Why does the error occur? This phenomenon occurs because NAT is not working properly. In most cases this is because routers / firewalls prevent ACKs from passing through. In this situation, the SIP type B timer comes into play, effectively interrupting the call when the 32 seconds expire. The settings of these Timers can probably be compromised by an incorrect NAT setting at the PBX level or at the telephone extension level.
In these cases, it is advisable to run a Firewall Test with the appropriate 3CX functionality and search for the presence of SIP ALG or incorrect NAT configurations.
