Is there a way to hook into the incoming phone call event and extract the number?
No, for security reasons you can't get info about incoming calls or SMS.
To the best of my knowledge.. no.. This is because the "phone" app works within its own sandbox and therefore nothing from other applications can extract data out.. Also, I'm pretty sure its against the Terms and conditions of the appstore, so Apple wouldn't approve it.
It may be possible on a jailbroken phone, however I imagine it would be difficult to do..
Sorry!