Configure legacy ISDN on R1 and R2. Use network 18.104.22.168/30.
Configure EIGRP AS 12 using MD5 authentication on both routers and advertise the ISDN network.
Redistribute EIGRP-12 into OSPF on R2.
Redistribute connected into EIGRP 12 on R1 for all devices to learn loopback(s).
Redistribute connected into OSPF on R2 for all devices to learn ISDN network 22.214.171.124/30.
Very tricky question. To enable redundancy for R1 to use ISDN if the outside interface of PIX is down, the only solution is to configure a GRE tunnel between R1 and R3, and configure the R1 BRI interface as the backup interface for the Tunnel interface, as demonstrated in the following example:
interface Tunnel3 ip address ...