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Can you call a taxi for us?... Puede llamar un taxi por nosotros (o para nosotros) I think por is correct because we are asking someone to call a taxi on our behalf.
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Yes, "por" is correct but "para" is correct too. The meaning is not the same:

- Llamar un taxi para nosotros: to be used by us.

- Llamar un taxi por nosotros: on our behalf.
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¿Nos puedes llamar un taxi?