You could do this with javascript.

I'm using this script to detect if it is mobile device: http://www.abeautifulsite.net/blog/2011/11/detecting-mobile-devices-with-javascript/

There are other javascript mobile detecters that detect more browsers but this one checks the most popular ones, just to clarify.

You need to add the javascript into the main template so it'll be reachable by every page.

You can find the templates by logging into umbraco and going to the 'Settings' section at the bottom.

Then at the left column you have the templates, now you add the javascript below to the main template (mostly there is only 1 main template but there could be more, just saying):

<script>
var isMobile = {
    Android: function() {
        return navigator.userAgent.match(/Android/i);
    },
    BlackBerry: function() {
        return navigator.userAgent.match(/BlackBerry/i);
    },
    iOS: function() {
        return navigator.userAgent.match(/iPhone|iPad|iPod/i);
    },
    Opera: function() {
        return navigator.userAgent.match(/Opera Mini/i);
    },
    Windows: function() {
        return navigator.userAgent.match(/IEMobile/i);
    },
    any: function() {
        return (isMobile.Android() || isMobile.BlackBerry() || isMobile.iOS() || isMobile.Opera() || isMobile.Windows());
    }
};

if (isMobile.any()) {
    // redirect to mobile site
    window.location = "yourMobileSiteUrl";
}
</script>

This will then check on every page if it's a mobile device and when it is it'll redirect to the mobile site.

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