One of the most common error situations in the context of VoIP SIP telephony concerns the sudden drop of calls at 32 seconds. This is a case related, in over 95% of cases, to problems or errors at the NAT configuration level.

To understand why this type of error occurs, it is necessary to introduce the topic of SIP Timers.

SIP Timer and call drop at 32 seconds.

SIP Timer T1, B, and F are used to primarily determine how long it takes for the remote device to respond before the sender considers it a timeout.

The 32 seconds is a rather common call drop interval and is substantially dictated by the action of the so-called “SIP Timer”. Here’s what happens in detail.

SIP Timer Types: T1, B and F

The T1 timer is the estimated round trip time of an IP packet and defaults to 500ms for most SIP systems.

Timer B is the maximum time the sender will wait to receive an INVITE. It 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. It corresponds to a value of 64 times the value of T1.

B and F are doubled on 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 – sends the 4th INVITE.
  • 4000 ms – sends the 5th INVITE.
  • 8000ms – send the 6th INVITE.
  • 16s – send 7th INVITE.
  • 32s – send the 8th and last INVITE. Call dropout occurs.

This explains the reason why a call dropped at 32sec.

Why does the error occur?

This phenomenon occurs because NAT is not working properly. In most cases this happens because routers/firewalls prevent the transit of ACKs. In this situation, the type B SIP timer comes into play, effectively interrupting the call after 32 seconds.

The settings of these Timers can probably be compromised by an incorrect NAT setting at the PBX level or at the telephone extension level.

It is advisable, in these cases, to run a Firewall Test with the appropriate 3CX functionality and look for the possible presence of SIP ALG or incorrect NAT configurations.