My Grandmother won't accept that I'm not a believer

First of all, it's worth saying that my grandmother is INCREDIBLY stubborn, doesn't like to/won't listen to people expressing a different view to her own and will start world war three over the tiniest thing. She's also very generous and caring (and although she would have absolutely none of it, mentally unstable) which is why I haven't turned my back on her.

She's always been very religious and very vocal about her faith. Over the years she has always tried to make me a believer and she believes its her mission from God to make me and the rest of the family Christians. I've always told her that I'm not interested and whilst I respect her beliefs, she must also respect mine. It's just getting to a point where I can't deal with having to listen to half an hour lectures about God, with her not listening to me. I don't want to cause an argument or raise my voice at all, because honestly it would be more trouble than it's worth, but I really don't know how to get her to stop and it's very much grating on me now! Any advice or tips are very much welcome!

Edit: I mostly speak to her over the phone as I live in a different city