Wait for digit while dial()

I want an incoming caller to be able to press 1 to leave a voicemail message while an operator is beeing Dial()'ed.

In the documentation this should be made possible by using the ‘d’ flag in the Dial() command. However, when I configure this it does not seem to have any effect at all. It seems like Asterisk it completely ignoring DTMF tones during the Dial().

Any ideas?