This is an Asterisk Support question, not General discussion about the AsteriskNOW installer.
You need to read the documentation for the gateway, to see if it allows some sort of pause digit.
Are you sending the PSTN digits as early media, or has the gateway prematurely answered. In the latter case, you should be able use the D option on Dial. I don't know any way of automating the sending of digits in the early media stage.