Use the following guidelines when using ISDN PRI non facility associated signaling in a survivable core server environment.
Whenever possible place both D-channels in one media gateway.
If it is not possible to place both D-channels in one media gateway, place the D-channels within media gateways where:
The media gateways are most likely to failover to the same survivable core server.
Enable Force Phones and Gateways to Active Survivable Servers?
feature on change system-parameters ip-options
screen.