Asterisk DISA: no dialtone provided or DTMF collected

If you are getting no dialtone with Asterisk DISA, and no DTMF is collected either, probably you need to put an "Answer()" in the dialplan before calling DISA. Otherwise, the channel is not established where DISA is supposed to play the dialtone or collect DTMF digits. I found that The Definitive Guide did not have "Answer()" in their examples of DISA regarding SLA.