You could cause the phone to stop functioning if you make a mistake doing this process, so take your time and make sure you know what you are doing. One phone that is known to work in this manner are phones manufactured by the LG Corporation. Make sure you do this on your own phone as some cell phones will work using this method and some will not. There are several freeware programs on the internet that will do the job for you, so do a Google search on dynamic IP addresses and take your pick of the programs available. After your phone comes back up use a laptop or a desk top computer to configure a dynamic IP address that your phone will use. Hit OK and then your phone should reboot. Then reset the home page on your phone to Google or Yahoo or something similar. Once you have entered a new proxy setting, press OK and then clear. You'll also need to reset the port on the phone by going to web settings and changing the port number to 8080. You'll want to change it to your own proxy settings. Go to the web settings and then proxy address settings to change the primary and secondary addresses.
If you have a mobile SIM card for a cellular phone and want to hack it for free internet, open the phone, then go to the menu, and then press 0 which will open up the service menu, the code is 00000.