You need to call and change your plan and you may need a new Sim.
In order to make phone calls using your tablet.... you will need to download a 3rd party app.
but as tmo_chris already stated... if you are using a tablet ... it already has unlimited texting on the plan you described.... provided that you have a tablet that is capable of doing texting. I believe all of the tablets that are T-Mobile branded are capable of text. If you are trying to use anything else... you need to check the manual of your device to see if it has texting capabilities.