After registering a card and adding funds for RingResponse billing, you can purchase phone numbers. To do so:
Navigate to More > Settings > Phone Numbers.
Click the green Buy Numbers button to get started
In the Number Store, you can search for toll-free numbers or local numbers within the US and Canada. You can add numbers to your cart and checkout to purchase the numbers using the card you registered for RingResponse billing.
You can find additional information on searching for numbers and purchasing numbers for multiple states here.