1. Go to Marketing > SMS > Numbers and click Buy New Number.
2. Under Plan choose One SMS Number: $12/mos or One Number with call forwarding and Greetings: $20/mos with 300m.
3. Enter a Title (a name to recognise the line, e.g. "Main line"). For the call-forwarding plan also fill Call Forward To, Greeting Text and optionally Redirect SMS?.
4. Click Next. On the Search available numbers page type an Area Code (e.g. 305) and click Search.
5. Pick one of the Available Numbers (US local numbers that support both SMS and voice) and click Buy Number.

For the basic plan the $12/month SMS Number add-on is charged to your card on file immediately and the number becomes Active. For the call-forwarding plan the number is reserved and the page shows "Request being processed" while the Sprwt team completes the setup (see the related question). You need the Edit Numbers permission to create numbers.