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Tom Arbuthnot MCSM Communications

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Join a PSTN Call Requiring DTMF Menu Tones To a Lync Conference

Published 21/04/2014 - 3 Comments

This comes up occasionally, so I thought I would document the workaround

On a normal Lync conference/Online meeting, you can add a remote telephone user by hitting “invite more people” and simply entering the number.



However, in this scenario, you cannot “Send DTMF tones” to the party joining the call. DTMF tones are when you press your telephone numbers to choose options from a menu, such as 1 for sales, 2 for accounts.

Sometimes you may want to bring a call into a conference that requires DTMF, for example, a language translation service or support line. There is a simple workaround, create a separate call, which does send DTMF tones, and merge it into the conference.

I have outlined the steps below in a quick Video

Tom Arbuthnot

Tom Arbuthnot

Principal Solutions Architect at Modality Systems
Tom Arbuthnot is Principal Solutions Architect at Unified Communications specialist Modality Systems. He is a Microsoft Certified Master and MVP, blogger, has a regular podcast with UCToday at and is a regular speaker at events including Microsoft TechEd and Ignite. He co-runs The Microsoft UC User Group London.


soder - 22/04/2014 Reply

I have noticed a strange behavior back in the OCS 2007 / R2 days: if you had an OCS audio conference, where the organizer of the OCS conf invited a PSTN bridge number into the conf to save costs by avoiding everybody individually dialing into the PSTN bridge OCS, that PSTN participant still remained in the conf after everybody -even the OCS conf organizer- left the OCS conf session. The result was, that call was still running as an active call from the OCS MCU to the PSTN destination number via a PSTN gateway, which resulted in a very expensive 8-10 hours call -espcially if the PSTN bridge was an international number. The only solution was to kill that particular call on the PSTN gateway (if its software allowed such an action), or we had to restart the AV MCU or the PSTN gw to force the drop of that session. PSTN conf bridges did not seem to have a maximum call length limit configured on their side to drop stuck calls. Another option was to put a max. call length limit in the PSTN gw config, but if you set that value too small, it resulted in legitime calls to be dropped as well (calls, that were indeed so long).

I dont know if Lync would still allow such an orphaned PSTN call-leg in an Audio conf for unlimited time while there are no Lync participants in the same conf room (as from the point of Lync it can still be a valid call scenario even if it wouldnt not make sense from a human point of view to have 1 person sittinh in a confcall with no other person). This is also a different call scenario from the unauthenticated participant joining to a dialin conf room, where after certain amount of time the Conference Attendant service will kick out the orphaned PSTN participants.

Tudor A. Lascu - 05/07/2018 Reply

thank you very much for sharing this.

Broken link for this video?

Best Regards,
Tudor A. Lascu

    Tom Arbuthnot - 09/07/2018 Reply

    yes sorry, I think the youtube channel I had this on is gone, hope the idea helped though,

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