Why is DTMF-pound not recognized as a valid DTMF input?
The pound character (#) is the default termination key for signalling an end to a DTMF-sequence entry. Putting the DTMF-pound in your grammar does not change this, so when a user enters dtmf-#, the system terminates the DTMF sequence. To change this default behavior, you can change the termination key.