Does Twilio queue accepted messages when carrier congestion delays downstream delivery?

For current Twilio Messaging API versions, the documentation describes rate limits and sender pools, but the internal queue retention behavior during carrier congestion is not clearly bounded. When an outbound campaign pushes messages through the API, the application often receives a 202 Accepted response before the message is actually handed off to the carrier. The unresolved behavior concerns the internal Twilio queue that holds these accepted requests during downstream carrier congestion: specifically, how long the queue retains a message before it is either processed or dropped, and whether that retention window is visible to the sender.

Because rate limits are dynamic and vary by geographic region and sender type—short code, toll-free, or long code—the point at which the API begins returning 429 Too Many Requests responses is not fixed. A Messaging Service can distribute traffic across senders, but it does not bypass the per-sender capacity constraints. This makes it difficult to distinguish between application-side dispatch latency, Twilio-side queuing, and carrier-level congestion using delivery timestamps alone.

Does the delta between the API request timestamp and the sent webhook callback reliably capture the full internal queuing duration, or does it include carrier-side delays that are outside Twilio's control? What observable signals—such as status callback ordering, error code 20429 frequency, or Monitor logs—indicate that an accepted message is still inside Twilio's queue rather than already in transit? Can the retention window be inferred from webhook latency patterns without triggering additional rate-limit pressure?