Have a look at a device called iO-Play as that offers exactly what you want in terms of functionality, you pair both the TomTom and your phone to it and it handles all the audio via the cars speaker system and it has it's own microphone. I have been using this accessory for nearly two years and it's never let me down, I gave up using the TomTom for hands free use after a number of crashes (of the TomTom not the car) it's simply too unreliable. In addition phone calls can only be routed via the TomToms built in speaker which isn't very good if the radio is blasting out, the iO unit mutes the radio during a call as well as the nav instructions, it returns back to the car radio/ CD post call or after a nav instruction.
Try a search on this forum for iO-Play and you should find a thread where I detailed fitting the unit to my car, I can't provide a direct link at the moment as I am mobile away from the PC, what model of car and year is it as you will probably need an addapter cable but these are easy to obtain from e-bay or even let Halfords fit it to your car. The unit retails at ?150 fitted by Halfords but can be found for under ?50 on e-bay, I recently purchased one for a mate and it was ?46 include postage - Mike