Ensure that the line is clean enough for transmission requirements. Shield the cable, if necessary.
Make sure that the cable is within the recommended length (no more than 50 feet [15.24 meters] or 25 feet [7.62 meters] for a T1 link).
Make sure that the local and remote CSU and DSUs are configured for the same framing and coding scheme as that used by the leased-line or other carrier service. One example is the Extended Superframe (ESF) and Binary 8-Zero Substitution (B8ZS).
If there is a problem with the framing and line coding configuration, then path or line code violations will be present in the output of the show controllers t1 command