> ## Documentation Index > Fetch the complete documentation index at: https://plivo.com/docs/llms.txt > Use this file to discover all available pages before exploring further. # Carrier Failover > How Plivo ensures uninterrupted outbound calling with automatic carrier failover * **What:** Automatically reroutes outbound calls through an alternate carrier when the primary route fails, is congested, or degrades * **How:** Fully automatic for most countries. No configuration or code changes needed * **India exception:** Indian regulations tie numbers to specific carriers, so failover works only on numbers that have a fallback number configured — enable **High Availability (HA)** to provision a backup number from a different carrier * **Coverage in India:** Voice API calls and ZenTrunk domestic India trunk calls * **Visibility:** Voice API hangup callbacks include `CarrierFailoverTriggered=true` when a failover was attempted on the call * **Cost:** HA adds a second number rental at the standard India local number rate (billed monthly in addition to your primary number) * **Limitation:** Applies to outbound calls only. Incoming calls are not covered by failover How Plivo ensures uninterrupted outbound calling with automatic carrier failover. Carrier outages, network congestion, and route degradation can disrupt outbound calls. Carrier failover eliminates this risk by automatically rerouting calls through an alternate carrier when the primary route is unavailable — keeping your calls connected without any interruption. *** ## How It Works Plivo maintains multiple carrier routes for every destination. When you place an outbound call, Plivo monitors the call route in real time. If the primary carrier fails or degrades, Plivo reroutes the call through an alternate carrier instantly. | Scenario | What happens | | ----------------------------- | --------------------------------------------------------------------------------------------- | | **Carrier outage** | Primary carrier is down or unreachable — calls reroute through an alternate carrier. | | **Network congestion** | High call volume causes drops on the primary route — calls shift to a less congested carrier. | | **Route quality degradation** | Call quality drops below acceptable thresholds — calls switch to a higher-quality route. | This is fully automatic — no configuration, no code changes, and no manual action required. Failover is transparent to both your application and your customer. India calls behave slightly differently — a retried call shows the fallback caller ID (see below). *** ## India — Additional Setup Required For most countries, Plivo handles carrier failover entirely behind the scenes. India is different. Indian telecom regulations tie phone numbers to specific carriers. Plivo cannot silently reroute calls through a different carrier's number without explicit provisioning. To get failover coverage for India numbers, you need to set up **High Availability (HA)** numbers. ### What Are HA Numbers? High Availability pairs your primary India number with a backup number from a different carrier. If the primary number or its carrier becomes unreachable, calls automatically reroute through the fallback number — no code changes needed in your application. 1. You enable HA on an India local number (new or existing). 2. Plivo provisions a second number from a different carrier and adds it as an active rental on your account. 3. Plivo monitors the primary number in real time. If a failure or congestion is detected, calls reroute through the fallback number automatically. Failover covers both Voice API calls and ZenTrunk domestic India trunk calls (calls placed from an India number to an India destination). In both cases, if the first carrier attempt fails, Plivo retries the call once through the fallback number instead of failing it. ### What Happens on a Retried Call 1. **Your customer sees the fallback caller ID**, not the original `from` number, on the retried call. 2. Plivo generates **one CDR and one hangup callback per call** — the retry isn't reported as a second call. 3. Retried calls take slightly longer to connect or fail than first-attempt calls. 4. On a retried ZenTrunk call, the CDR's `from_number` records the fallback caller ID, not the number you originally sent. ### When Plivo Retries a Call Plivo retries a call only when the first attempt fails because of a carrier fault — not when the failure is a genuine call outcome. | First attempt result | Retried? | | ---------------------------------------------------------------- | -------- | | The carrier rejects or can't complete the call | Yes | | The called party is busy, or the caller cancels the call | No | | The called party is temporarily unavailable after ringing starts | No | | The called party is temporarily unavailable with no ringing | Yes | | The request times out | Yes | ### Track Failover on Your Hangup Callback Voice API calls report failover through the `CarrierFailoverTriggered` parameter on the [hangup callback](/docs/voice/api/calls#callbacks) (`hangup_url`). Plivo sends `CarrierFailoverTriggered=true` only when a failover was attempted on the call. The parameter is omitted otherwise — never sent as `false` — so existing integrations and the normal payload shape are unaffected. The Call Details API doesn't return this field. ```text Example hangup callback (failover attempted) theme={null} CallUUID=aaaaaaaa-bbbb-cccc-dddd-eeeeeeeeeeee Event=Hangup CallStatus=no-answer CarrierFailoverTriggered=true ``` ZenTrunk has no webhook callbacks. To identify retried trunk calls, check the CDR's `from_number` for the fallback caller ID. ### Enable HA When Buying a New Number 1. Go to **Phone Numbers > Buy Number** in the [Plivo console](https://cx.plivo.com/phone-numbers/search/). 2. Search for an India local number. 3. On the Details page, scroll to **High Availability**. 4. Check **Enable high availability backup number**. 5. Complete the purchase — Plivo provisions a fallback number and bills it automatically. ### Enable HA on an Existing Number 1. Go to **Phone Numbers > Active Numbers** in the [Plivo console](https://cx.plivo.com/active-phone-numbers/). 2. Click the number you want to upgrade. 3. Scroll to the **High Availability** section. 4. Check **Enable high availability backup number** and click **Save**. 5. The **Fallback Number** appears in your number details once provisioned. ### Billing Enabling HA rents a second number from a different carrier. This fallback number is added as a separate active rental on your Plivo account and billed at the standard monthly rate for India local numbers — in addition to your primary number. The fallback number is visible under **Phone Numbers > Active Numbers** alongside your other rentals. *** ## Frequently Asked Questions ### Do I need to change my application code? No. Carrier failover is automatic and transparent. Your application continues to use the same Plivo APIs and number configuration without any changes. For India HA numbers, failover is also automatic once you enable it — no code changes required. *** ### How can I see which fallback number is assigned to my India number? Go to **Phone Numbers > Active Numbers** in the [Plivo console](https://cx.plivo.com/active-phone-numbers/) and click on the primary number. The **Fallback Number** field shows the assigned backup number. *** ### Does carrier failover cover incoming calls? Carrier failover applies to outbound calls. Incoming call behavior follows your standard number configuration. *** ### Do I need HA numbers outside India? No. Plivo handles carrier failover automatically for all countries other than India. HA numbers are only needed for India due to regulatory requirements that tie numbers to specific carriers. *** ## Related * [India Calling Regulations](/docs/voice/concepts/india-calling) — Regulatory requirements for voice calling in India * [India Number KYC](/docs/numbers/rent-india-numbers) — KYC and number rental process for India * [Phone Numbers](/docs/numbers) — Overview of number types * [Account Phone Numbers](/docs/numbers/account-phone-numbers) — Manage your active numbers Questions? Contact [Plivo Support](https://support.plivo.com) or your account manager.