Unfortunately I do not think there is an easy answer to this, other than to say that the simpler you make your mobile site the more likely it is to appear correctly/consistently across multiple devices/networks. You also need to consider the 'app' mindset with iPhone, where something that previously might have been presented simply as a web site, is now frequently presented in an app instead. Finally, bear in mind that many phones do not support flash (iPhone famously..), silverlight, javascript etc. If your site loses too much of its look and feel without these than you may have to redirect to targeted sub-sites per phone type.
See this similar question which may help you get a feel first for how your web site looks on different devices and in different networks:
http://stackoverflow.com/questions/3022574/testing-mobile-sites/3023848#3023848
If you do need to change the look and feel for different phones, then you will need some way to detect the phone types - there are many articles etc about this which you should be able to find with Google - for example:
http://www.hand-interactive.com/resources/detect-mobile-php.htm
Again, it would be good to test whatever one you use in your target network to check it works as expected (operators may use devices, for example proxies and optimisers, which may affect how things arrive at your webserver or phone...).