Graham The xtgem "click to sms" block just adds a preset phone number and no text in the body
The problem with sms: urls is it seems there are 3 different options for how to add body text using
?body= ... for android and most older mobiles
Then just because Apple want to be different
iOS 5 may not work at all
;body=... for iOS 6 and 7
&body=... for iOS 8

Some old mobiles do want the body data url encoded and some do not
Also some will not work without a number being declared so I have placed a 0 in the number field
This script is a test to produce a sms with a pre filled body and no number on as many devices as possible
I am interested to see if it will work on real iOS devices and others as I only have emulators

<script> var ua = navigator.userAgent.toLowerCase(); var url = "sms:0"; var is_os8 = (ua.indexOf("os 8") > -1) ? "&" : ";"; url += (ua.indexOf("iphone") > -1 || ua.indexOf("ipad") > -1) ? is_os8 : "?"; url += "body=" + encodeURIComponent("Hey check this out <xt:url />"); document.write('<div><a href="'+url+'">Text this page to your friends</a></div>'); </script>

See http://blog.julianklotz.de/2015/03/14/the-sms-uri-scheme/