Switching sim card into different cell phone?

ok so i have 2 phones both from tmobile. i just got the new sidekick from tmobile in June, but i kept my old phone. then i heard if u switch the sim card into my old phone it will then work again. i did this and it worked. is this bad for my phone, or will it damage it. i know that is what the workers at the phone... show more ok so i have 2 phones both from tmobile. i just got the new sidekick from tmobile in June, but i kept my old phone. then i heard if u switch the sim card into my old phone it will then work again. i did this and it worked. is this bad for my phone, or will it damage it. i know that is what the workers at the phone store do when u get a new phone. will my billing be the same, like i have texting, will i be charged if i text on my old phone but put my sim card into it. also is this bad to do or against the law?
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