feat: overhaul call error docs with fault classification and troubleshooting guide

[dhruva-reddy]

Description

• Rewrote fern/calls/call-ended-reason.mdx — Complete overhaul of the call ended reasons reference page:
  • Added "Understanding error prefixes" section explaining vapifault/providerfault/pipeline-error-* fault classification and billing impact
  • Added "Quick diagnosis" table mapping caller-experienced symptoms to error categories
  • Added ~40 missing error codes: subscription/billing errors, transfer/warm-transfer errors, SIP-specific errors (403/407/408/480/503), infrastructure errors (worker-died, transport-never-connected), and call hook errors
  • Removed 5 phantom values that don't exist in the codebase (assistant-error, assistant-not-provided, database-error, unknown-error, call.in-progress.error-sip-telephony-provider-failed-to-connect-call)
  • Fixed incorrect pipeline-error-* explanation (was misleadingly described as BYOK-only)
  • Reorganized from flat h4 headings into logical hierarchy with actionable descriptions
  • Added "Next steps" section with cross-links
• Created fern/calls/troubleshoot-call-errors.mdx — New symptom-oriented troubleshooting guide organized by what the caller experienced (6 impact buckets):
  1. Phone never rang (billing, config, server URL, infrastructure)
  2. Phone rang but nobody answered
  3. Call dropped mid-conversation (worker crashes, transport disconnects)
  4. Assistant went silent or unresponsive (LLM, voice/TTS, transcriber/STT errors)
  5. Transfer failed (general, SIP, telephony provider)
  6. Call ended normally (expected end reasons with tuning guidance)
• Updated fern/docs.yml — Added navigation entry for the new troubleshooting page under Calls, directly after "Call end reasons"
• Updated fern/debugging.mdx — Added cross-link callout after "Common error patterns" table pointing to both the reference page and the new troubleshooting guide

Testing Steps

• Run fern check — passes with 0 errors (4 pre-existing warnings unrelated to these changes)
• Verify internal links resolve to existing pages
• Confirm new page renders at /calls/troubleshoot-call-errors in preview deployment
• Confirm navigation entry appears in correct sidebar position (under Calls, after "Call end reasons")
• Review page content for style guide compliance (active voice, present tense, no marketing language)
• Verify cross-links between all three pages work bidirectionally

[github-actions]

🌿 Preview your docs: https://vapi-preview-85b83c94-612c-4238-8d9e-0e7500833cae.docs.buildwithfern.com