Where: Settings → SMS.
One-time setup
A2P is a carrier requirement
US carriers require every business SMS sender to register a brand and a campaign, via Twilio. It isn't optional and it isn't something PortaPro can skip. Settings → SMS walks you through registration and shows current status (pending / active / rejected).
How new numbers get their webhooks
Provisioning a Twilio number through Settings → SMS wires the inbound and delivery-status webhooks automatically. There's nothing to configure per number.
Costs and limits
Twilio's per-message costs and A2P registration fees pass through. See Settings → SMS for current pricing.