SIP Timers T1, B, and F are used to determine mainly how long it takes for the remote device to respond before the sender considers it a timeout. 32 seconds is a very common number to see for things for audio drops, and there’s a reason why – SIP timers. Here’s what’s going on:

T1 is the estimated round trip time of an IP packet, and defaults to 500ms. Timer B is maximum time sender will wait for an INVITE to be acknowledged and Timer F is the maximum time sender will wait for non-INVITE messages. These typically default to 64 times the value of T1. B and F are doubled each iteration so an INVITE with no response looks like this:

  • T 0 – send original INVITE
  • 500ms – send 2nd INVITE
  • 1000ms – send 3rd INVITE
  • 2000ms – send 4th INVITE
  • 4000ms – send 5th INVITE
  • 8000ms – send 6th INVITE
  • 16s – send 7th INVITE
  • 32s – send 8th and final INVITE

That’s where 32 seconds comes in for a lot of people who say “my calls last 32 seconds and then drop.” It’s because NAT is not working correctly and firewalls prevent the ACK messages so SIP timer B comes into play and drops the call after it thinks it has timed out.

These timers should be able to be specified for all initiatiors of SIP messages (phones, PBXes, SIP speakers, etc).  If you are having 32 second audio problems, start by looking at firewalls and NAT settings, because it is likely something there that is causing your SIP timers to expire without proper responses.