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My company sells/supports Allworx SIP PBXs. We have a weird edge case with one of our customers who use PRI for dial-tone. On outbound calls, if a user happens to receive an SMS on their iPhone, and the iPhone is set to play the default "note" tone, and the tone is loud enough to be heard over the handset or speakerphone, outbound voice is muted. This is reproducible 100% of the time. It only happens on outbound calls through the PRI, and only on the specific note alert tone. If I route calls out their backup SIP trunks, it does not happen. Packet captures show that the outgoing voice is audible as far as the LAN port of the PBX, so it must get muted upstream.

The PBX connects locally to the carrier's Adtran TA900e using PRI. The Adtran converts it to SIP and is registered with an Adtran TA908e on the far side (according to the carrier, anyway.)

After research and arguing with the carrier (they believe it is a physical problem with the PRI card), I think the most likely answer is an iPhone alert causing talk-off triggering DTMF tones which triggers some sort of * code that mutes the call.

So, my questions are:

I've experienced DTMF talk-off myself when talking to my wife on our cell phones, but is it possible for a specific smartphone alert to trigger talk-off?

If yes, could one of the Adtrans (or anything upstream) have a feature enabled that lets DTMF mute outgoing media?

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Yes we experienced an Asterisk issue years ago where DTMF received over a trunk would cause Asterisk to drop the call. It's been long enough that I don't recall the fix but it was hell troubleshooting to find why the calls were being dropped. Usually from an accidental keypress on a cell phone while driving.

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  • Was the call dropped or muted?
    – pooter03
    Mar 5, 2020 at 14:19
  • Also, this is definitely happening upstream of our PBX as it doesn't happen with any other carrier. We've also been able to recreate the issue with other customers using this carrier.
    – pooter03
    Mar 9, 2020 at 19:35

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