Which method guarantees zero downtime when migrating a Twilio number to a new application?

Answer the exact question first

The method that guarantees zero downtime when moving a Twilio number to a new application is the Twilio Number Migration API. It is an asynchronous, account‑internal operation that keeps the number reachable while the old application remains active until the migration status turns Completed.

Why this method works

• Both the old and new applications stay online during the transfer.
• Twilio automatically updates the number’s Voice and Messaging URLs to point to the new application’s endpoints once the migration finishes.
• The process is fully managed by Twilio, so no manual webhook changes are required during the cut‑over.

Steps to perform a zero‑downtime migration

1. Prepare the new application
   • Create the new application (or Studio Flow / Flex worker pool) in the same Twilio account.
   • Deploy the new code and verify that the Voice URL and Messaging URL are reachable from the public internet.
   • Optionally, test inbound call and SMS against the new application’s endpoints.
2. Check current number configuration
   • In the Twilio Console or via the API, confirm that the number’s Voice and Messaging URLs are valid and that the number is not currently attached to a Studio Flow or Flex worker pool that will need re‑association.
3. Initiate migration

   # Twilio CLI example – replace {APPLICATION_SID} with the new app’s SID
   $ twilio api:core:applications:assignments:create \
     --application {APPLICATION_SID} \
     --phone-number +1234567890
   

   Or use the REST API:

   POST https://api.twilio.com/2010-04-01/Accounts/{AccountSid}/Applications/{NewApplicationSid}/Assignments.json
   Content-Type: application/x-www-form-urlencoded
   Body: PhoneNumber=%2B1234567890
   

4. Monitor migration status
   • Poll the assignment resource until the status field reads Completed.
   • During this window the old application continues to receive traffic, so no interruption occurs.
5. Post‑migration cleanup
   • If the number was part of a Studio Flow or Flex worker pool, re‑associate it with the new application’s resources.
   • Delete or archive the old application’s webhook URLs to avoid accidental traffic.

What to do if the number is in a Studio Flow or Flex

After migration, the Voice URL will still point to the new application, but any Studio Flow or Flex configuration that referenced the old application will need to be updated manually. Re‑link the flow or worker pool to the new application’s SID to maintain full functionality.

When to consider a different approach

• If you must move the number to a different Twilio account, the process requires a support‑initiated request and can take several days; it does not guarantee zero downtime during the transfer window.
• If your application is split across multiple accounts or you cannot keep the number in the same account, the staged webhook update method (pointing to a staging endpoint and then switching) can be used, but it carries a higher risk of mis‑configuration and is not as reliable as the Number Migration API.

Ask for missing diagnostic detail

To ensure the recommendation fits your exact setup, could you confirm whether the phone number is currently linked to a Twilio Studio Flow or Flex worker pool? If so, we’ll outline the exact re‑association steps after migration.