Assume that you join a conference call that's hosted through the Audio Video Multipoint Conferencing Unit (AVMCU) by using Microsoft Lync Server 2010, Microsoft Lync Server 2013, or Microsoft Skype for Business Server.
In this scenario, you cannot add attendees to the conference if those attendees require Dual Tone Multi Frequency (DTMF) tones to join the conference.
This issue may occur in the following situations:
To connect the attendee to the conference, you have to enter the extension of the attendee by using the Lync client keypad or the Skype for Business client keypad.
Outgoing calls from your enterprise go through a carrier that requires the caller to enter a passcode for accounting.
Outgoing calls to a remote interactive voice response (IVR) system require a keypad entry.
The AVMCU feature in Lync Server 2010, Lync Server 2013, and Skype for Business Server does not support DTMF relay.
To work around this issue, join the Lync or Skype for Business conference by using Skype for Business or Lync client, and then make a peer-to-peer call to the remote party that requires DTMF tones.
After you connect to the remote party as a peer, merge this call into the conference call. To do this, select the ellipses (...) on the right-side of the existing Skype for Business call window, select Merge This Call Into, and then select the conference call conversation that you have joined.