How does texting charity donations on your cell phone work? (See below)?

When there's a disaster, I often get texts requesting money for the Red Cross, etc. Can you donate $20 (or any amount) by responding to the text, and then does your mobile phone carrier bill you at the end of the month? Or do they bill you right away? I've always sent checks as donations, but it would... show more When there's a disaster, I often get texts requesting money for the Red Cross, etc. Can you donate $20 (or any amount) by responding to the text, and then does your mobile phone carrier bill you at the end of the month? Or do they bill you right away? I've always sent checks as donations, but it would simplify the process if I could just do it that way. Thank you.
Update: I want to understand the process. Does my phone company bill me right away, or do they add it at the end of the month? (I understand only to donate to charities I know are real or on the up and up). Thanks, again.
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