mobile phone as a transmitter?

12V @ 900MHz isnt really informative, it doesnt meant anything
what power level do you want as an output ? 1W, 10W, 50Watts ?

Dave

Yes...it can be used but with a lot modifications, and the power output will depend on the output module which may not be more than 1 watt. As in practical, mobiles communicate with towers which are equipped with more sensitive receivers, the low power of the handset will work fine.

What is your signal? What is the modulation? What specification such as IMD3 do it need?

Normally the mobile mximum output power is 0.5W for protecting human body, that is for voice. But you need power is (12V**2)/50R=2.5W. So I don't think so.