Node-red will be running on a server somewhere and waiting for some input. So you will need some app on the phone that will send a message to node-red.
From your question, I'm guessing that you haven't much experience with NR so I'd recommend watching this playlist: Node-RED Essentials. The videos are done by the developers of node-red. They're nice & short and to the point. You will understand a whole lot more in about 1 hour. A small investment for a lot of gain.
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