To allow an additional number (DID) to be set up on your account, contact Support and we will make the changes to your subscription for you. New numbers are $2/month and are prepaid in advance to match your subscription.
This means if you had an annual plan and you set up an additional number slot with the subscription it would be $2 x 12 months, or $24.
Getting a new local number assigned to your new number slot is .99 cents to set up.
Additional numbers can be removed from your account at any time and the unused months will be refunded to your account.