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

Goal

Move a small Twilio-based telephony application from its current deployment to a new codebase while keeping the same phone number and avoiding any service interruption.

Constraints & Uncertainty

Twilio phone numbers are bound to the Account SID that owns them; transferring the number to another Twilio account requires a support‑initiated request and can take several days. Incoming call and SMS routing is controlled by the Voice URL and SMS URL configured on the number (or via a Messaging Service). Updating these webhooks is instant but not transactional, so a mis‑configured endpoint can cause immediate failures.

Unresolved Decision

Should the migration keep the existing Twilio account and simply swap the webhook endpoints, or is it preferable to transfer the number to a new account and then re‑configure the routing? The choice depends on the expected support turnaround, regulatory requirements, and the ability to run parallel instances of the old and new application.

Specific Questions

• What is the exact timeline and documentation required for an account‑to‑account number transfer in the US, and how does it affect the ability to perform a zero‑downtime cutover?
• Can a staged webhook update—pointing the number’s Voice/SMS URLs to a staging endpoint while the old application remains live—guarantee that no inbound traffic is lost during the switch?
• Which Twilio features (e.g., TwiML Bin, Studio Flow fallback) can serve as a safety net to absorb traffic if the new endpoint fails during the transition?