What do you mean by "get sync to work"? I am not familiar with all the (including lack of) features for '09 Sync, but the system should do everything it is capable of without a phone connected, besides allowing for hands-free phone calls or things that require cellular data which is nothing related to the operation of the vehicle functions.
Backing up a bit, why can't you do it on your flip phone? If it has bluetooth, and the capability of pairing with another device, it may be able to provide all the functionality that your sync system is capable of. A smart phone cannot control the sync system, just use it as a peripheral like you would a wireless headset/mic, or dialer with the phone as the client to make calls. I could be wrong, not having '09 era sync but I doubt they had more features back then that they removed later.
Is it that you just prefer a flip phone or is it very old? If it is very old, you may lose network connectivity on it soon because many carriers are phasing out not only 3G but also 4G models that don't have LTE. If that is the case, it's not a choice of whether you want to keep using your old phone because it will stop working and may have already lost a significant # of towers it can connect to, so areas where you get a usable signal become fewer and fewer.