Phoning the ISP
So you've opted to gather all your debugging information before calling. You know which sorts of errors are appearing on your circuit, and what sort of packets your circuit will and will not pass. Now what?
You got your T1 circuit from either a telco or an ISP. Phone them. The first words out of your mouth should be, "Hi, I have a down circuit." They will ask you for your circuit ID, customer number, or other identifying information. When you reach a technical support person, explain:
The state of the line itself (both line and protocol down, just protocol down, or both line and protocol up)
That you have rebooted your router and CSU/DSU (this will almost certainly be their first suggestion)
The types of errors you are seeing: CRC, ...
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