Let’s start with --> can you elaborate your whole problem in a better way?
From what i understood, You want to basically route your SIM800’s Microphone and Speaker to a Bluetooth device, in this case it is supposed to be car’s audio system.
so to answer that, i would say , you have to find a Bluetooth headset with a capabilities of mic and speaker , now disconnect the mic and speaker of the Bluetooth device and connect it to the Mic --> Speaker of SIM800 and Speaker --> Mic Of the SIM800.
Well all this is totally hypothetical but the concept may work with some better research.
Not to mention that, you just cannot use a sim800 without a good MCU and a Best supporting firmware to answer the calls and all.
My personal recommendation to you would be to rather buy the Cheapest phone which you can even get for like 900Rs or less, and use it. you don’t need a smart phone. and save the time and efforts!