Short answer: build an omnichannel “unified inbox” using either a dedicated omnichannel helpdesk or a custom integration layer that funnels messages from each app into one ticketing/agent UI, then add routing, automation, and analytics. Below is a practical, step‑by‑step plan, technical options, and a checklist you can follow.

1. Quick audit (1–3 days)

• List every channel you currently use (email, web chat, in‑app chat, SMS, WhatsApp, Facebook/Instagram DMs, Twitter/X, phone/VoIP, support forms, marketplace messages).
• Note whether there are native integrations (e.g., Facebook Pages, Twilio for SMS/WhatsApp), webhooks/APIs, or manual exports.
• Identify volume per channel, peak hours, and typical workflows (who replies, escalation paths).

2. Decide approach (two common paths)

• Off‑the‑shelf omnichannel helpdesk (fastest, least engineering):
  • Examples: Zendesk, Freshdesk, Intercom, Front, Help Scout, Gorgias (e‑commerce), Chatwoot (open‑source).
  • Pros: built‑in integrations, unified inbox, reporting, SLAs, macros, bots, authentication, support.
  • Cons: recurring cost, sometimes limited customization.
• Custom integration layer (more flexible, needs engineering):
  • Use message queues, a central ticket store/DB, and a single agent UI (or connect to existing CRM).
  • Use middleware/no‑code for simpler builds: Zapier, Make, n8n.
  • Pros: full control, cheaper at scale, custom routing.
  • Cons: engineering time and maintenance.

3. Integration basics (how messages are centralized)

• Connect channels via:
  • Native integrations in your helpdesk (preferred).
  • Webhooks → your backend service that converts incoming payloads to a ticket/message format.
  • APIs polling (if no webhook).
  • Middleware (Zapier / n8n / Make) for apps without direct integrations.
• Normalize messages into a common structure: customer id, channel, timestamp, message text/attachments, metadata (conversation id, language).
• Map identities: match by email, phone, or external user id to create a single customer profile and conversation history.

4. Routing, automation & UX

• Unified inbox: single view where agents see all channels and can reply from the same UI.
• Routing rules: by skill, product, language, priority, or SLA. Use round‑robin or skill‑based assignment.
• Macros/templates for common replies; canned responses and snippets.
• Automated triage: bots/answer recommender to handle FAQs, then escalate to human.
• Bot → human handoff rules (time, topic, or sentiment triggers).

5. Data, tracking & quality

• Track key metrics: First Response Time, Average Handle Time, Time to Resolution, CSAT, First Contact Resolution, backlog.
• Tagging/labels for root‑cause analysis.
• Dashboards and weekly reviews.

6. Security & compliance

• Enforce RBAC for agents.
• Encrypt data at rest and in transit; review vendor SOC2/GDPR/CCPA status if needed.
• For regulated channels (WhatsApp/phone) ensure consent and retention policies.
• Audit logs for changes and message access.

7. Historical data & migration

• Export history from current apps (CSV/JSON) and import into the unified system as tickets or customer notes.
• Preserve message timestamps, attachments, and original channel metadata.
• Run migration on a copy first and verify integrity.

8. Testing & rollout

• Pilot with a small agent group and a subset of channels for 1–2 weeks.
• Collect agent feedback and adjust macros, routing, notifications.
• Train agents on the new UI and playbooks.
• Full rollout with monitoring and a rollback plan.

9. Example architecture (custom)

• Channel integrations (webhooks/API) → Ingestion service → Message normalizer → Message queue → Router/service rules → Ticketing DB + unified agent UI → Notifications/agent actions → Outbound APIs to each channel.
• Optional: add CRM sync for customer 360, knowledge base, and analytics pipeline.

10. Recommended stacks by scenario

• Small startup / low budget:
  • Chatwoot (open‑source) or Help Scout / Intercom starter + Zapier for niche channels.
  • Timeline: 1–3 weeks.
  • Cost: low to moderate per agent.
• E‑commerce with high order volume:
  • Gorgias (shopify‑centric), Zendesk Suite, or Front + native marketplace integrations.
  • Add automation for order lookups, refunds, and returns.
  • Timeline: 2–6 weeks.
• Midmarket / enterprise:
  • Zendesk/Freshdesk/Fabinbox/ServiceNow or a custom integration to Salesforce Service Cloud.
  • Add advanced routing, workforce management, and SSO/SCIM.
  • Timeline: 6–12+ weeks.

11. Common pitfalls to avoid

• Centralizing without identity resolution → fragmented history.
• Too many notifications → agent burnout; consolidate and use batching.
• Over‑automating transfers without fallback to human.
• Ignoring compliance or consent, especially for messaging services like WhatsApp.

12. Minimal first‑week action plan (practical)

• Day 1: Inventory channels and export sample data.
• Day 2: Select a platform or middleware (trial accounts).
• Day 3–4: Connect 1–2 high‑volume channels (email + web chat).
• Day 5: Configure unified inbox, a few macros, and routing for a pilot agent.
• Day 6–7: Run pilot, collect feedback, iterate.

13. Success metrics to watch first 90 days

• Reduction in response time (target: 30–50% faster).
• Increase in CSAT or NPS (baseline vs 90d).
• Percent of messages handled via automation vs human.
• Drop in duplicate conversations and escalations.

If you want, tell me which channels you need to centralize, your team size, and budget and I’ll give a specific platform recommendation and a 30/60/90‑day rollout plan.