For US/Canadian accounts that have completed registration, outgoing texts are sent through your registered messaging service rather than directly from an individual number; the service picks one of the numbers attached to it. In the current version the From dropdown on Compose, Chat and Sequences is still recorded with the message (and is required — you get "No SMS Number Selected" without it), but the carrier-facing sender is chosen by the messaging service. Accounts outside the A2P countries send directly from the selected number.