This is an example of a T1 configuration on a Cisco 2500 with IOS 10.3 (yes I know the router is old as dirt). The router does not have a csu/dsu built in so you will need one to hook the line from the phone company up to. Ask your Internet provider for the line settings of the csu/dsu like framing type (ex. b8zs) and line coding type (ex. ESF).Also you need to see what channel your starting on. If your PBX is going to use 12 of the 24 channels then your csu/dsu needs to start on channel 13. If your using a full T1 then you start a channel 1.
Your Internet provider should also be able to provide the encapsulation type (ex. IETF) and lmi-type (ex. ansi). They will give you a block of IP addresses and tell you what is the gateway for your router. For my example its the serial0 line below. For the internal gateway for your network you will be using another interface. The example uses Ethernet0. The first IP on your block of IP's is usually binded to this interface. This is the gateway for the computers on your network.
This example of course has fake IP's so you'll need to subsitute your own. The config below is output from the "show config" command at a router prompt. But they are also the commands you need to do when you are enabled and in config terminal mode. If anyone wants a better description of this mail me (see the "about" section for the e-mail address) and maby I'll put more detail here.
version 10.3 ! hostname blah.test.org ! enable secret 5 (#$30u34(#(%)@_$%*#!_BIEWQBMX enable password passwordhere ! ip subnet-zero ! interface Ethernet0 ip address 126.96.36.199 255.255.255.128 no ip directed-broadcast ! interface Serial0 description Gateway to Internet provider ip address 188.8.131.52 255.255.255.252 no ip directed-broadcast encapsulation frame-relay IETF no ip route-cache bandwidth 1536 frame-relay lmi-type ansi ! interface Serial1 no ip address shutdown ! ip classless ip route 0.0.0.0 0.0.0.0 184.108.40.206 banner login ^C -=Welcome to the main gateway=- ^C ! line con 0 password passwordhere login line aux 0 transport input all line vty 0 4 password passwordhere login ! end