Problem: Message Waiting Indicator (WMI) from external VM

I have Asterisk 11.8 running with a Digium 4 port PC card for use as a call router/blocker in my home. My phone service is ATT PSTN and I need to keep my external ATT voicemail. When a call is unanswered it will go to the ATT voicemail. Prior to deploying my call blocker I would get a message waiting indicator (MWI) on my phones and a stutter dial tone if the message is unheard. The stutter dial tone is not a critical issue, but the MWI is. Does anyone know of a way for me to make the WMI work when using an external voicemail system? I think I know what has to be done, but I am unable to find any documentation on how to script it:

  1. After an unanswered call, check that all extensions ( or maybe DAHDI channels?) are on hook
  2. If they are on hook, check the provider (connected to FXO port) for a stutter dialtone
  3. If there is a stutter dialtone, set WMI on the appropriate DAHDI channel.
    Any suggestions or insight is greatly appreciated.