100rel/PRACK: failure, bug or missing feature?

I have an old Yealink T26P on a slightly flaky internet connection. On an outbound call, the 183 Progress packet was lost. The user hung up after hearing ~30 seconds of silence and complained about the failed call.

I set Retransmission to Enabled on the phone, but it didn’t work right. On incoming calls, the phone sends provisional responses reliably (good), but on outgoing, pjsip doesn’t send them reliably (bad), even though the phone’s INVITE properly includes Supported: 100rel and Allow: PRACK.

I was able to work around the problem by adding to pjsip_custom_post.conf:


What is wrong here? The default value is 100rel=yes (confirmed by pjsip show endpoint) and I would think that if both sides say it’s supported (optional) it should be used. If there is a good reason why not, I would think that there should be a setting in the GUI to turn it on.

Anyone successfully using 100rel on outgoing calls?

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