Use native TwiML to isolate caller transport during capacity tests

When Twilio load tests fail before the caller WebSocket connects, a native TwiML Say/Pause caller can remove the public tunnel and caller STT/TTS from the capacity lane. This helps separate test-generator failures from limits in the deployed voice service. Keep interactive callers as a separate quality lane: native callers follow timed speech and cannot establish that they heard a correct response or completed a business action. Count fresh provider activity against attributed backend roots, and retain uncertainty where exact call mapping is unavailable.

Applies when: a bidirectional Media Streams generator adds uncertain transport failures or speech-provider load to a voice capacity test. Keywords: Twilio TwiML Media Streams tunnel capacity caller speech observation