Telnyx is one of the four SMS providers Assistant works with. The account, the number, and the balance are yours; Assistant only needs two pieces of data.
1. In the Telnyx portal. Create your account at telnyx.com, buy an SMS-capable number, and create a Messaging Profile for it. Generate an API Key in the API Keys section and add funds. Two things that bite: a toll-free number requires completing its verification before it can send and only reaches the United States, Canada, and Puerto Rico; and every country you want to send to must be enabled in your profile's Allowed destinations. The documentation is at developers.telnyx.com/docs/messaging.
2. In Assistant. Go to Settings → Notification - File and pick the telnyx provider.
Paste the API Key and the sender number exactly as Telnyx gave it to you, with its country code.
3. Save and test with SEND TEST SMS.
If something fails. "Toll-free number not yet verified": complete the verification in the portal. "Number outside enabled regions": enable that country in Allowed destinations. Error 40001: the customer's number is a landline and cannot receive SMS, ask for a mobile. And if you send outside your country, add it in MANAGE COUNTRIES too.