Using your cell phone with the Onstar hands free speaker/mic

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can't u just take out the Onstar system (after free yrs..) and then find speaker/mic wires and then somehow make them as plugs (of those handsfrees out on the market) and then hook it up to the phone..?
i think this would work.. since handsfrees r just extended mic/speaker system..
well.. can someone verify on this?
im not an engeenier