First you can browse your existing numbers.
If you have no spare numbers you can buy one.
You can purchase a number using the Nexmo CLI. The following command purchases an available number in the US. Specify an alternate two-character country code to purchase a number in another country.
nexmo number:buy -c US --confirm
Make a note of the number returned, as you'll need it later.
Connect to a WebSocketConnect an inbound call to a WebSocket with the Voice API