Funny, I just took a look at what I figured must be the last four BRI backups anywhere and started troubleshooting why they didn't work, when they dial their PRI headend. I see the exact same issue, its interesting because initially the call connects but from what it looks like the call is not capable of passing data (if you disable authentication it will change the message but will not change the behavior). I am going to work on this tuesday, I think I have solved this issue before and if I remember correctly this was related to the number on the PRI headend terminating via switched termination rather than dedicated. I will let you know what I find.
In the mean time you can make sure that your remote end can pass data by making a loopback connection (no need to do anything physically): just use the command isdn call interface to call yourself, if you have debug ppp negotiate and debug isdn q931 you should see the call setup go out and then come right back from the ISDN switch, it should proceded and the router should even try to authenticate with itself (depending on your exact setup the 2-way auth will likely fail but it proves you can send data to the local teleco and back).
