Cancel in-progress DTMF collection

**Rate limit: Per command** · **Burst:** 4 · **Refill:** 0.8 req/s Cancel the current DTMF collection for this session. If no collection is in progress, the request is accepted and has no effect. If a collection is in progress, cancellation proceeds in these stages: 1. The `202` response accepts the cancellation request. 2. Prompt playback stops promptly. 3. The collection phase ends according to its existing timers. 4. The platform emits the terminal `digits.collected` webhook with `status: cancelled`. The `202` response does not mean that the terminal webhook is ready. Input entered after cancellation is discarded from later collections. It is reported only by the cancelled command's terminal `digits.collected` event. This DELETE uses its own rate-limit bucket. The corresponding POST uses the same configuration in a separate bucket. See [Keypad input lifecycle events](/api/keypad-input#lifecycle-events) for complete payload schemas and examples.