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Some carriers have messaging apps that might allow for this. Beyond that I'm not sure something like that is out there (outside of some jailbreak tweak).
 
I mean I doubt if there is such an app you asked for, at least I am not aware of such app which can answer the calls and texts for you, AI (artificial intelligence) robots can do this heheehe
It seems the question was more in the sense of an app/service that simply auto-responds with some sort of a predefined message to any incoming messages (no AI or anything of any real complexity would be needed for something like that).
 
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