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Re: Mobile Phone app
It's not clear from your question whether you cannot answer the phone because you hear the phone ring but you're doing something else and it's not convenient to answer, or whether you're perhaps asleep or not near the phone when it rings so unable to take the call.
I guess a start might be to switch voicemail off, but Android has a feature with a series of quick response messages you can use when you hear the phone ring and don't wish to answer a call.
A quick Google found the following;