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