Often times you can use the carriers website.... Along with the recipient's phone number to send a text w/o using a callback number.
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Depends on where you are. If in the USA, go to your local Walmart and buy yourself a Tracfone. Register it from a telephone number neither at your home or workplace, and text away. In the rest of the world, buy a SIM card, or steal someone else's if the local government forces you to register. Then text away.
603-668-1884 call me I will tell you how to do anything you need!