The Android App can use either your phone's SIM card number or you can send through a Business Texter Virtual Number.
Purchase a virtual number by purchasing a plan online or through the Play Store.
Once the plan is in place, click into the Buy Credits screen and select "Get Number". A pool of numbers from your area code will be displayed that you can pick from.
On Android Tablets without a SIM card, you must have a virtual number selected to send and receive texts on the tablet.