---
title: "Prefix overrides"
slug: "prefix-overrides"
description: "Bulk lookup of phone numbers to ensure they are valid before using them for outbound dialing. Choose either the Number profile or Line activity option."
tags: ["Dialer", "campaigns", "Mobile number validation", "Validation", "Number lookup", "Number validation"]
updated: 2026-07-16T16:38:43Z
published: 2026-07-16T16:38:43Z
canonical: "docs.voiso.com/prefix-overrides"
---

> ## Documentation Index
> Fetch the complete documentation index at: https://docs.voiso.com/llms.txt
> Use this file to discover all available pages before exploring further.

# Prefix overrides

**Who should read this article**: Administrators

Rewrite the destination (dialed) number of outbound calls by prefix before each call is placed. Define pairs of a **Number prefix** to match and an **Override prefix** to replace it.

 
            Note
  
      

The **Prefix override** page appears under the **Outbound** menu only when the feature is switched on in **Administration > System settings**.

  

Refer to [Features tab](/v1/docs/features-tab) for how to switch the feature on.

## Introduction

The **Prefix override** feature enables you to modify the destination number of outbound calls by matching and replacing its leading digits. For each outbound call, Voiso removes the longest matching prefix from the start of the dialed number and prepends a replacement prefix in its place, then routes the call as usual. The rewrite happens before the call is placed and before billing, so the call is routed, recorded, and rated against the rewritten number.

The Prefix override feature is available for all contact center plans and applies to all outbound call types, including [click-to-call](/v1/docs/click-to-call) and [Dialer campaigns](/v1/docs/outbound-dialer-campaigns-overview). Any user who can access **Administration > System settings** can enable the feature and manage prefix pairs; no separate permission is required.

  
            Note
  
      

Prefix override changes only the destination number (the B-number). It never changes the caller ID (the A-number) presented on the call.

  

## The Prefix override page

To manage prefix pairs, from the Voiso navigation bar, navigate to **Outbound > Prefix override**. The page displays a table of prefix pairs with the following columns:

| Column | Description |
| --- | --- |
| Number prefix | The leading digits Voiso matches at the start of the dialed number. When matched, these digits are removed. |
| Override prefix | The digits Voiso prepends in place of the matched number prefix. May be empty. |

## How prefix rewriting works

For each outbound call, Voiso compares the start of the dialed number against every **Number prefix** in the table and applies the rule as follows:

1. Find the **longest** number prefix that matches the start of the dialed number.
2. Remove that number prefix from the start of the number.
3. Prepend the paired override prefix.
4. Place the call using the resulting number.

The rewrite is anchored to the start of the number, and it is applied only once — Voiso does not re-check the rewritten number against the table.

For example, with a pair that maps number prefix `54` to override prefix `549`, dialing `541381377587` places the call as `5491381377587`.

When more than one number prefix matches, the longest one wins. With pairs `541` and `54138`, dialing `541381377587` uses the `54138` pair, because it is the longer match.

  
            Tip
  
      

To exempt a narrower range from a broader rule, add a pair whose **Override prefix** equals its **Number prefix**. For example, with `54` mapped to `549` and `5412` mapped to `5412`, numbers starting with `5412` are placed unchanged, while other numbers starting with `54` become `549…`. This works because the longer, matching prefix wins and rewrites the range back to itself.

  

## Adding a prefix pair

To add a prefix pair, follow these steps:

1. From the Voiso navigation bar, navigate to **Outbound > Prefix override**.
2. Click **Add** to display a new pair row.
3. In the **Number prefix** field, enter the leading digits to match and remove.
4. In the **Override prefix** field, enter the digits to prepend. Leave this field empty to remove the matched prefix without prepending anything.
5. Click **Save**.  

The following rules apply to both fields:

- Values may contain digits only — no letters, symbols, special characters, or spaces.
- Each value may be at most 50 digits long.
- **Number prefix** must contain at least one digit.
- **Override prefix** may be empty. An empty override prefix removes the matched number prefix and prepends nothing.
- Each **Number prefix** must be unique. Adding a second pair with a number prefix that already exists is rejected.

## Editing a prefix pair

To edit a prefix pair, change the values in its **Number prefix** or **Override prefix** field, then click **Save**. Changes take effect on the next matching call.

## Deleting a prefix pair

The rewrite is anchored to the start of the number, and it is applied only once — Voiso does not re-check the rewritten number against the table.

For example, with a pair that maps number prefix `54` to override prefix `549`, dialing `541381377587` places the call as `5491381377587`.

When more than one number prefix matches, the longest one wins. With pairs `541` and `54138`, dialing `541381377587` uses the `54138` pair, because it is the longer match.

  
            Tip
  
      

To exempt a narrower range from a broader rule, add a pair whose **Override prefix** equals its **Number prefix**. For example, with `54` mapped to `549` and `5412` mapped to `5412`, numbers starting with `5412` are placed unchanged, while other numbers starting with `54` become `549…`. This works because the longer, matching prefix wins and rewrites the range back to itself.