If you put your SIM card from your old phone into your new iphone 3G or 3GS you got from eBay will you be charged any kind of fee for upgrading to a new phone?
No, however you must keep in mind that AT&T requires a special data package for iPhone to be added on your account. Without this additional package, the iPhone will not function on AT&T's network. ...
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Can you keep your t-Mobil Phone number after switching your sim card and phone?
You should be able to. There is something called the phone number portability act.
Do you know all your phone numbers by heart in your cell phone or do you just hit the name?
No. I'm bad with phone numbers