Bluetooth enabled light, trigger by cell phone calls

I need a compact battery powered bluetooth device that will automatically illuminate a light (possibly an LED) when my cell phone is in use (voice call). I know that many Bluetooth headsets already have battery powered LEDs on them. I would like to keep functionality for the microphone and speaker on the phone, but use Bluetooth to trigger a light in a remote location that stays lit for e duration of my phone call. Need help with device design and iPhone firmware/app.