Free PBX: Outbound calls via LQ fail when a SIP-secret is configured

By Andreas Schick
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    Question: Calls to longer outbound numbers via LQ often fail when Free PBX is configured with SIP secret enabled. Why does that happen?

     

    Answer: On a Free PBX install make sure you allow TCP as well as UDP for SIP.

     

    Explanation:

    The reason for that behaviour is the SIP stack used use inside the LQ.

    Whenever a SIP packet exceeds the size of 1300 bytes it will be sent out via TCP instead of UDP by the SIP-stack inside the LQ.

     

    Now when a SIP secret is configured on Free PBX, Free PBX will challenge the LQ to send the Auth data, by replying to the first INVITE (which is typically sent without the Auth data) sending a 401 Unauthorised. This results in a second INVITE (now with the Auth data) to be sent from the LQ.

    This second packet is likely to exceed the 1300 bytes limit for UDP, especially if longer phone numbers are called (e.g. international phone numbers longer than 12 digits).

     

    Free PBX needs to see that second TCP INVITE, otherwise the LQ will never receive the proper response to that INVITE and the call will eventually time out.

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