Are you using a regular, wired, landline phone, or a wirelss/cell phone?
Landline phones ring signals are driven by a voltage pulse. You can detect that pulse with an appropriate Zener diode, and tie that detection to a blinking circuit.
Cell phones do not typically have a standard signal that tells you someone's calling, although you could maybe get that information out of the Bluetooth interface if it's supported. (This is approximately 100,000 times more complicated than the landline version!)