For those with an auxiliary jack, you can plug your phone into it then talk by using your speakerphone and car's sound system.

I'm assuming a lot of people may already know this, but thought I should post it anyway. It's better than using just your speakerphone because if your phone is like mine, it cannot overcome that sweet sound coming from behind

 

Do you know if the 07 997 C4s has an ancillary jack?

 

None of the 997's had auxillary jacks between 2005 and 2008. It was an option starting in 2009 on the 997.2.

To get an auxillary jack on a 997.1, you need to install a Dension Gateway or equivalent device.