DTMF signal played over what exactly? The phone option is for when you aren't at a computer (driving, conference, etc..).
A and B are on a phone call. A starts a video meeting. B goes to shortlink.dtmf or opens the app, which starts listening. A clicks "transmit room code" which goes over the existing phone connection. B's client hears the signal, decodes it, and gives them a link to join.