Is there a way to trigger a workflow when a certain DTMF sequence occurs?

Use case is as follows:

User A is on a call with B. A wishes to transfer the call to C. A presses ** and enters a code, which is a combination of a call transfer "FAC" and the number they want to transfer to (in this case User C).

I would anticipate that we would use the ** trigger to activate other call features, most likely those in the Engagement Call Control api.

How can I get the workflow triggered upon pressing the **?

Thanks!

Seyhan