Call Control and Mobility Troubleshooting
Domain 3 - Call Control and Mobility Troubleshooting
About 15% of the exam is diagnosis: read evidence, isolate the failing layer, and name the fix. Expect questions built around traces, the Dialed Number Analyzer, RTMT, and SIP/ISDN cause codes.
The toolset
- Dialed Number Analyzer (DNA) simulates digit analysis for a chosen device, partition and CSS - it reports the matched pattern and the resulting transformations but places no real call and generates no media.
- RTMT provides real-time counters, alert logs, trace and log collection, and packet capture.
- Traces: SDI shows the signaling messages exchanged with gateways and trunks (SIP, ISDN); SDL shows internal call processing. The default trace level is Error, so you must raise it to Detailed before reproducing the fault.
- CDR/CMR via CDR Analysis and Reporting shows how a completed call was dispositioned and its cause value.
Reading failures
No matching pattern gives a reorder tone and cause 1 (unallocated number) in the trace. SIP 401/407 is a normal digest-authentication challenge, not a failure. 404 means the number is unknown, 480 that the destination never answered, 486 that it was busy, 488 that no common codec or media resource existed, 503 that the destination was unreachable, 603 that the far end declined. Cause 63 with Call Rejected usually points at authorization rather than routing.
Worked example
Extension 4102 dials 8-555-0142 and hears fast busy, while 9-555-0142 works. DNA run as 4102 shows no pattern match for 85550142. The 9.@ route patterns sit in partition PSTN-ACCESS, which the phone's CSS contains; the translation pattern 8XXXX sits in partition SITE-B, which the CSS does not contain. The trunk was never involved - the failure is a CSS/partition miss. Adding SITE-B to the CSS fixes it. The SDI trace confirms the diagnosis: the Digit Analysis step reports no pattern match, not a gateway rejection.
Traps that cost marks
- Analyzing a default-level trace and concluding nothing happened; raise the trace level first.
- Treating a 401 as the root cause when it is the expected first step of digest authentication.
- Assuming a fast-busy tone means trunk failure - most often it is a missing pattern or CSS membership.
- Overlooking region/codec mismatch and MRGL exhaustion when the failure is 488 rather than a routing error.
- Trusting DNA to prove media works; it only proves digit analysis.