What is Dual DTMF?

Dual-tone multi-frequency signaling (DTMF) is used for <a href="https://en.wikipedia.org/wiki/Signalling_(telecommunications)" target="_blank" rel="nofollow noopener noreferrer">Signalling (Telecommunications)</a> over analog telephone lines in the voice-frequency band between <a href="https://en.wikipedia.org/wiki/Telephone" target="_blank" rel="nofollow noopener noreferrer">Telephone</a> handsets and other communications devices and the <a href="https://en.wikipedia.org/wiki/Automatic_telephone_exchange" target="_blank" rel="nofollow noopener noreferrer">Automatic Telephone Exchange</a>.

(Source: <a href="https://en.wikipedia.org/wiki/Dtmf" target="_blank" rel="nofollow noopener noreferrer">Dtmf</a>)

Where is it used? : It's used in Customer Care calls made to Call Centres all around the world.

However, I'd like to know what the term "Dual DTMF" means? Tried searching the internet, but couldn't figure out the exact usage it provides:

Here's a useful #-Link-Snipped-#. Let me know.