When trying to bring up a WAN connection (forgetting PPP and Frame Relay for the moment), you could use the OSI model:
Layer 1 – Check the cable to ensure that it is attached correctly. Has the no shut command been applied? Is there a clock rate applied to the DCE side?
RouterA#show controllers serial 0
HD unit 0, idb = 0x1AE828, driver structure at 0x1B4BA0
buffer size 1524 HD unit 0, V.35 DTE cable
RouterA#show ip interface brief
Interface IP-Address OK? Method Status Protocol
Serial0 188.8.131.52 YES . . .
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