Once the Phone channel is enabled on your account you can purchase a local numbers for over 40 countries and toll-free numbers for US & Canada. To purchase a number go to Admin -> Phone and click on Buy Number. There you would be given the option to choose toll-free or local and the country of your preference. The option to purchase the number would appear next to the number of your choice - kindly click on that to complete the process.
P.S - If you are in the Sprout please please ensure that the card details are added under Accounts -> Plans and billing -> Enter payment details so that you would be able to make the purchase.