MsgOrb sends from the phone number attached to the WhatsApp Business Account you connected. You do not choose it in MsgOrb and there is no number picker in the app.
How the number is resolved
When you save credentials, MsgOrb asks Meta for the phone numbers on your WABA and stores the first one's Phone Number ID. Everything MsgOrb sends afterwards uses that ID.
If your WABA holds more than one number and you need a specific one, open Advanced Setup → Manual WABA Configuration (WhatsApp API Credentials) and enter that number's Phone Number ID by hand alongside the WABA ID and token.
Where the number is shown
The WhatsApp Number Health card on WhatsApp Settings names the number Meta is reporting on. If that is not the number you expect, the credentials point at the wrong WhatsApp Business Account.
Test numbers
A Meta test number (the free one Meta issues with a new app) only delivers to the handful of recipients you allowlist in Meta Dashboard → WhatsApp → API Setup. Sends to anyone else fail with Meta error 131030, and MsgOrb reports *Recipient is not in your Meta test allowlist*.
Template approval still works normally on a test number. Only delivery is restricted.