Using my existing phone number on my tablet

Tablets don’t have the software needed 

If your plan allows the use of your phone's hotspot you can tether the tablet to your phone. Otherwise you would need to add a second line at the tablet rate to use it with cellular (assuming it's a cellular device)

If your plan does allow the use of your phone's hotspot, you connect the tablet to the phone just as you would any other Wi-Fi connection. Your phone's hotspot has a passcode to allow devices to connect to it