I've copied the xhtml page:

<?xml version="1.0"?>
<!DOCTYPE html PUBLIC "-//WAPFORUM//DTD XHTML Mobile 1.0//EN"
"http://www.wapforum.org/DTD/xhtml-mobile10.dtd">
<html xmlns="http://www.w3.org/1999/xhtml">
<head><title>My Page</title></head>
<body>
<h1>My Name</h1>
<p>Welcome to my page.<br/>
1 <a href="myfile2.xhtml" accesskey="1">See my next page</a><br/>
2 <a href="wtai://wp/mc;805551212" accesskey="2">Call Joe</a><br/>
</p>
</body>
</html>

from the Nokia XHTML guideline document. I've tried text/html, application/xhtml+xml and application/vnd.wap.xhtml+xml for the response code, but when i try to download the page on the Nokia6650 i get the error message File format unknown. The strange thing is that it works on the Nokia3300 and Nokia6600