I have been given the task of sending messages (SMS/Flash/ other formats) to any phone entering a restricted area... How do I detect the phone and send message to it without any prior knowledge of it's SIM Number?
If you rely on GSM technology then you need a server-based solution. You need a mechanism for registering devices on the server. The server queries the mobile networks location center for device positions and send a message if a position trigger fires. Position information may be less accurate for this solution.
If you have the means to do nearfield communication, e.g. WiFi or Bluetooth, you could build a solution on top of this. You would mark the restricted zone with NFC technology and have software running on the device looking for these markers. If the device detects such a marker this would trigger an appropriate action, maybe ask the mobile network for a message.