Spoofing is beyond my expertise, but you can use textnow to generate SMS capable phone numbers. They used to be excellent for sending textmessages 'anonymously' but now you have to download their bad app to obtain an account. As well as this they have a habbit of instant banning accounts that utilize proxies. I guess you could download an emulator, download the textnow APK, then generate an account using a residential proxy or try your luck with another proxy service. It takes effort but it is completely free. There might be better alternatives now days though.