Dial Rules for a SIP Trunk

In your outbound route, put something like this for 800 calls.
9|18XX.

This matches on 9-1800-xxx-xxxx, but does not send the 9 to the actual trunk. Only the digits sent after the "|" are sent to the actual trunk unless you put something in the trunk form (like the outbound dial prefix section) which inserts something for all calls sent to this trunk.

For example our SIP provider whats 1+area code + number even for 10 digit local calls, so we set up two trunks, identical to each other, except the "local" version inserts the 1 in the outbound dial prefix section of the trunk. We then point 9|NXXXXXXXXX toward the Local SIP and 9|1NXXXXXXXXX to the LD SIP trunk which does not insert the 1.

Have you tried setting up a softphone like x-lite?
Try using it to access your IVR internally.
Go to CLI and watch as you access the IVR both from outside, softphone internally, and hard phone internally.
If you do not get double digits, then would have to point toward your phone instruments or SIP provider.
Have heard of incorrect dtmf settings providing no tones, but double tones.

OK

nortelvoip,

Go into /etc/asterisk/logger.conf

And on your console= line add ,dtmf to the end. Should look something like:

console => debug,verbose,dtmf

Then go into the asterisk CLI (asterisk -r) and watch a call. You'll see the DTMF keypresses.