CPC (Calling Party Control) & Kewlstart Problem

I have a problem with kewlstart on my TDM400P that seems to be related to the CPC method of my provider.
The 3rd paragraph of sandman.com/cpcbull.html describes my problem exactly…

“There’s often a short open (0 volts DC) on a phone line just after you go off-hook, or just after you’ve finished dialing a phone number. These are usually very short opens, like 20 to 50ms. If your phone system Open Loop Disconnect timer is set at 50ms, you may never be able to make a call because every call would be cut-off as soon as you went off-hook or were finished dialing. That Open Loop Disconnect Timer is very important!”

In my case it’s when I’ve finished dialing a phone number. I’ve used a butt-set with polarity led’s on my lines and can see that there is a brief moment that no power is on the line just after DTMF tones are sent and before the other end rings. My Asterisk (Trixbox 1.1.1) system immediately disconnects the call when this happens. When I use loopstart the problem goes away. The problem with loopstart is I get long voicemails and I’m unhappy using busy disconnect because my provider does not go immediately to those tones, they play a recording several times before they do.

I am using the latest zaptel drivers (1.2.7) and have tried adjusting zt_kewltime in zaptel.h to 1000 with no luck.

Are there any other settings I can adjust?

I’ve contacted my provider and their only suggestion is to switch to groundstart, which I’m not sure would be the best answer.