You can access the antenna cable for the on-star, however because you would have to use a barrel splice and extend the cable to your phone, the gain would be almost non-existant. Your Nokia outputs approx. 400 milliwatts or 4/10 of a watt. Your splice, additional connectors and the added cable lenght would lower your output to approx 100 millis. There is a product that can help you. It is a mobile booster and is available for almost all phones currently on the market. Go to
www.smoothtalker.com for the info. This booster provides 3 watts of power in analog mode and a gain of 25db in digital. Depending on your phone model, they may also have a complete hands free install kit to integrate with the booster. Jay