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It in theory could do GSM, though you might need two, depending on if it can tune from the tower->handset and handset->tower frequencies fast enough.

There are plenty of relatively inexpensive chips that support LTE and are legal if you're actually interested in this.

Possible? Maybe at the cellular 3.84MS rate. Legal, no.

It looks to be half duplex, so probably not. At least not with a single board.