Doug and I worked this problem together, Doug was our Microsoft consultant at the time. I came up with the solution of a fake 180 ringing message and Doug had the skills to write the script to pull it off. We started out using it to solve a PSTN issue we were having with sim ring while a user was logged off but I have also used it to solve an R2 ringback issue when interoping with our Cisco deployment. Its a great article he has written.