As long as you swapped the SIM cards (You kept the SIM card you had the entire time, that you pulled out of your Sidekick) which would also allow you to keep your own number so when someone calls you at your number, it now rings on the new phone, nothing new will be charges to him unless he uses extra features he didn't have on his phone. For that same token, if he doesn't have the plan with data included for free, it will charge him each time he uses any type of data connection, such as the internet or anything similar (watching MobiTV, downloading games, etc..). Good luck and I hope this helps.