Cisco 2600 with FXO


I am using Asterisk as a PBX for my LAN (softphone SJphne) . Now i want to call on PSTN network. I have already Cisco 2600 router with 4 FXO ports with 4 PSTN lines.

Router and asterisk are on same LAN can ping each other

I have no idea how to forward all out going call from asterisk server to Cisco router.
Please give me some idea