The single most useful thing I can give you is the methodology itself, because once the structure is in your head you can apply it to any node or protocol. Here is the loop I use, mapped to your symptom to resolution flow.
First, frame the symptom precisely before touching anything: what, where, when, who, how many, and since when. Scope it to one cell, one subscriber, one APN, or the whole node. A vague symptom is the number one reason RCA fails.
Second, draw the call flow for that exact service before you diagnose it. List the nodes and interfaces the procedure crosses. For a VoLTE MO call that is UE to eNB to MME to S/PGW plus IMS P/S/I-CSCF and TAS, over Gm, Mw, Cx, Sh, Gx and Rx. You cannot localise a fault you cannot draw.
Third, gather evidence classified by layer and time: alarms, KPIs and counters, node logs, and a trace or pcap of one failed attempt. Align the timestamps first, because a mis-aligned trace invents a false story.
Fourth, walk the call flow message by message and find the first place it deviates: the first error response, the first missing message, the first reject. The failure is where the flow first breaks, not where the symptom appears.
Fifth, read the exact cause value at that break: the Diameter result-code, the GTP cause, the SIP response, the NAS EMM or 5GMM cause, the MAP or ISUP error. That code names the failing procedure and which side rejected.
Sixth, apply the counterfactual before you commit: a cause is only the root cause if changing that one thing would remove the symptom. Reject the explanation that needs the most special pleading.
Seventh, fix and verify at the same layer you diagnosed: symptom gone, KPI back to baseline, and re-run the trace to confirm the flow now completes. Watch for side effects.
To practice this for free, build a lab you can break on purpose. Open5GS or free5GC give you a full EPC and 5GC; add UERANSIM or srsRAN for the UE side. Capture with Wireshark, which dissects NGAP, NAS-5GS, Diameter, GTP, SIP and HTTP/2 SBI, and use sngrep for live SIP. For the correct flows to compare your broken traces against, read TS 23.401 for EPC, TS 23.502 for 5GC procedures, TS 24.301 and 24.501 for NAS causes, TS 29.274 for GTPv2 causes, and TS 24.229 for IMS. Break one thing, trace it, find the first deviation, fix it, and confirm. That single habit builds more skill than any document.
Happy to help as you go. If you want structured one-to-one walk-throughs of real traces, that is the kind of mentoring I do, so tag me on a specific failed call flow and we can work it end to end.