Inline Bluetooth adapter

Have a vehicle that doesn't have Bluetooth, the head unit is integrated with the climate control at the hardware level making it difficult to replace. Is there a Bluetooth adapter that could be connected inline with the rca line output from the radio then to an amplifier available? I need to use my phone not just to stream music but for calls the adapters I see just give you either an FM output which means I would have to leave the radio on a particular frequency which wouldn't allow me to use it normally or has an aux out which I would switch to when needed both these options wouldn't really be flexible. Need the radio to play normally then when a call comes in its muted and I can just press a button and answer the call.
Any suggestion would be appreciated.
Thanks.