A customer has a 3660 with two ISDN PRI interfaces with different carriers, hence two different number ranges. When a remote user dials in and requests callback, the callback fails half the time. If just one PRI is used the callback works, whichever carrier is used. I think the problem is due to the inherent security of ISDN.
If remote user dials in from "1234" to the router on "6789" and the callback goes back out on the same carrier, then the return call is from "6789" to "1234" and the call succeeds.
If the next call is from "1234" to "6789", the router uses round robin and selects the other PRI to make the callback. The return call will be from, say, "4567" to "1234" and because the numbers differ, the call is rejected.
I have got this right and if so, is there any way round it.