Can I use a macro lens to take pictures of large items?
 

I just got a 100mm Canon Macro lens for my 20D and was wondering if it was practical to use it for products (lets say like a telephone) that has different areas of focus? I gave it a try and it would only focus on the part at a single distance if you know what I mean, not sure the terms. What setting do I need to change on the camera to focus the whole product? Should I just use my 28-135 mm lens?

Probably.

Try aperture priority (Av) mode and set a higher value such as f8 or f11. The camera focuses with the lens wide open (f2 or f2.8 depending on which 100m macro you have) but will stop down when you take the shot. This will increase your captured depth of field, although I'm not sure it will be enough. Try it and see. Don't forget your shutter speed will be slower so you may need more light or a higher ISO setting.