Capturing IPs

Anyone aware of a program available to capture & display IP addresses in a closed network? My company is designing a wireless emergency VOIP phone system for the local bridge commission, and they want the ability to see on their computer which of the 12 phones is sending the call. I figured the easiest thing to do would be to capture the IP's of the VOIP routers at each phone, then maybe cross reference it to a databse to give a location name on the screen. Of course, I'm not a computer/networking guru...so what I'm thinking might be ridiculously easy, or very difficult...I don't know. We have I.T. guys at work who I can ask, but not on the weekend.