Sidekick-Slide sim card

I lost my MyTouch 2 weeks ago, and after calling T-Mobile and deactivating it, the clerk told me that I should get a new phone off of eBay to save money, and that if I went into the local store, they'd have a new SIM card for me.
Now that was fine and all, but when I went to go pick up my SIM card with my sister, one of her friends said that because it's an older phone, I'd need the entire setup for it to even get my SIM card.

I don't know what he meant by "entire setup", and I really want to know if this is true or not.

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I don't know what he means either All you would need is the sim card.