[VoiceOps] PRI with SIP trunk backup for incoming DIDs

I am looking for a carrier that will sell me an ISDN PRI to which I can assign blocks of DIDs. However, I need to route these DIDs into my equipment using alternate means should the PRI fail (local copper facilities in my area are unreliable). Ideally calls destined for these DIDs with a downed PRI would be routed over a SIP trunk. Simple "forward to another number if the PRI goes down" will not cut it. Possible?