Can boost mobile work with an unlocked GSM phone?

No. Boost mobile, along with Nextel are called iden. They look the same as gsm sims but are incompatible. Gsm sims of course are from Att, Tmobile and most of the rest of the world .

Then there's non sim phones like Verizon and Sprint. All of these technologies are mutually exclusive and incompatible with each other with a few rare exceptions.

Gsm sims will work in an unlocked Nextel i920/930 but iden sims won't work in gsm unlocked phones. Sorry.

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It's unlikely unless they give you a sim card. For example, if you walk into Verizon, they activate the phone. But if you buy prepaid, it comes off the shelf.