Detect mobile operator by number

It should be fluid lay-out, XHTML 1.0 Transitional is OK. Some phones only understand WAP, some only HTML. You should use HTML, WAP is not used very much and even the iPhone does not support it. Make the files as small as possible (minify them) and use small images. Use simple navigation and take a look at other mobile sites. The best for navigation should be a list (ul). Some examples are:

• http://m.nos.nl/ (dutch, however)
• http://hyves.nl/mini (also dutch)
• http://mobile.wikipedia.org/
• last.fm on your iPod Touch/iPhone
• mobile youtube
• mobile Google

Also, make a version special for smartphones/phones with big screens (like Blackberry storm or iPhone). They have more memmory and can handle more images etc... They often come with cheap internet or internet whit a static price (e.g. $10/month for infinite internet (that month)).

Most modern devices support the tel: scheme. So use <a href="tel:555-555-5555">555-555-5555</a> and you should be good to go.

If you want to use it for an image, the <a> tag can handle the <img/> placed in it just like other normal situations with : <a href="tel:555-555-5555"><img src="path/to/phone/icon.jpg" /></a>