Switch off cellular network off/on on iPhone (triggered by Bluetooth car kit)
On a jailbroken iPhone, with SBProfiles and SBSettings you can utilise profiles to group multiple actions to different situations. The downside is that you have to manually switch the profiles when entering and leaving the car.


Make sure you have SBProfiles and SBSettings installed from Cydia.
Open SBProfiles. It should have multiple profiles by default; Default and Car among them. Select Car and then Toggles. Drag Airplane and Bluetooth to On section. Make sure that Airplane is on top of Bluetooth, otherwise activation of Airplane will turn Bluetooth back off.
Then, go back to Car and select Done from the top bar. Select Default and then Toggles. Drag Airplane to Off section (and Bluetooth, if you don't need it outside the car).
Select Settings from the bottom tabs. Select Activation Method(s) under Profile Switching. Here you can select the action(s) that trigger the profile selection to become active. I think it defaults to Short Hold of the Sleep Button.


Now, when you hold the Sleep Button a bit longer than shutting down the screen but not long enough to power off the iPhone, you can select Car profile and have cellular turned on but Bluetooth on. Or select Default to turn cellular back on. I think you will have to make the call still, unless there's a way to automate it, too.