Auto-dial device too fast for Comcast voice line?

I have a ticket open with Comcast but I thought I'd check here also.

I have an elevator emergency phone, which requires a standard POTS line (FXS) and has a built-in auto-dialer.  I've connected this to a line off my Arris modem from which my Business Voice lines come, and it draws dial tone just fine.  However, when the device dials, I get a slow (regular) busy signal.  If I plug in a phone to the line in the elevator, I can dial out just fine (manually, so slower).

So, Comcast's line is not recognizing the DTMF (touch tone) signals from my elevator phone.

1) Has anyone else had this problem, and if so how did you resolve it?

2) Is there a "ringdown" configuration or service that I can get from Comcast that would eliminate the need for my elevator phone to do the dialing?

3) Does Comcast support out-of-band DTMF signaling that could be enabled (possibly on the Arris modem) to allow this to work?

This really is a Comcast problem, although there has been a lot of push-back on it.  I am just trying to get a little ahead of the curve here for when they call me back.

Thanks!