A few question about different phone

Does a phone need to have a sim card in order to work for ATT

Do only GSM phones work for GSM companies

Will a Sprint phone work for a company that uses sim cards.

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ATT is a GSM carrier, therefore only phones with sim cards will work, specifically ATT sim cards, or sim cards from a roaming partner. GSM is a different technology than what Sprint and Verizon use, they use CDMA. And in answer to your last question, for those reasons Sprint phones will not work with ATT.