What is a failover DID? How does it work?

What is a failover DID? How does it work?

A failover DID is a backup inbound phone number that automatically takes over when your primary number can’t accept calls, keeping you reachable through outages or congestion.

Table of contents

  1. Quick definition
  2. Why you might need one
  3. How failover routing works
  4. Set-up guide in <Your Portal>
  5. Testing & monitoring
  6. Edge cases & limitations
  7. FAQ

1 · Quick definition

Term

Meaning

Primary DID

Your main inbound number that rings first.

Failover DID

Secondary number that receives calls automatically when the primary fails to connect (busy, unreachable, SIP 5xx, etc.).

2 · Why you might need one

  • Business continuity – Maintain voice support during carrier outages.
  • Zero missed-call SLAs – Meet strict response-time targets.
  • Disaster recovery – Keep critical lines open during local power or bandwidth failures.

3 · How failover routing works

  1. Inbound call hits the primary DID.
  2. Your carrier performs a health check (SIP OPTION/PING or HTTP callback).
  3. If the response is 2xx, the call proceeds normally.
  4. If the response is busy, timeout, or ≥5xx, the carrier instantly reroutes to the failover DID.
  5. For the caller, the hand-off is silent; ring continuity is preserved.

4 · Set-up guide in

Prerequisites

  • Admin role in the portal
  • Two active DIDs on your account
  • A reachable failover destination (SIP URI, mobile, or landline)

Steps

  1. Numbers → Primary DID → Edit
  2. Enable Failover Routing → select your backup DID
  3. Choose failover trigger:
    • Timeout (e.g., 3 rings / 15 s)
    • SIP error codes (486 Busy, 503 Service Unavailable)
  4. Click Save → status changes to Protected within 30 s

5 · Testing & monitoring

Test

How to run

Expected result

SIP 503 injection

Temporarily block your PBX

Call diverts to failover DID

Ringing timeout

Let primary ring without answering

After X s, failover rings

Portal health dashboard

View Voice → Alerts

“Primary unreachable” event logged

Enable email/SMS alerts so ops staff know when failover is active.

6 · Edge cases & limitations

  • Failover applies to inbound calls only; outbound traffic still originates from the trunk you dial.
  • Some carriers charge a dual-billing fee when failover triggers more than N times/month.
  • Emergency (e.g., 911/112) calls ignore custom failover rules.
  • If both DIDs terminate to the same PBX, failover offers no resilience—place the backup on a separate route or data center.

7 · FAQ

Q: Can I assign multiple failover DIDs in a cascade?
A: Yes—add tiers in Numbers → Advanced Routing (primary → failover 1 → failover 2).

Q: Will callers notice longer ring times during failover?
A: Typically no; diversion happens at the carrier level in < 1 s.