I ran into this exact same problem. There is a problem with the alarmreceiver module and/or dsp.c module re: dtmf dialing. I had the module working for years with an old version of Asterisk but could not get the alarmreceiver module to work with Asterisk 10 or 11. However it works perfectly with Asterisk 1.8.18.0
I used a PBX in a Flash distro.
***One additional note: I also needed to force the extension to use the ulaw codec. Configure the alarm panel asterisk extension using Freepbx to the following:
dtmfmode - in band audio
disallow - all
allow - ulaw
FYI - I wrote up a very detailed procedure for the alarmreceiver module at the link below if interested: