Can Truecaller Android get Caller ID on 2G connection? Just curious. Truecaller emphasizes that it requires 3G/Wifi connection in order to tell caller ID? Does this mean that if I'm in area where only 2G is available and no Wifi is available Truecaller cannot get a CallerID for me?
People install Truecaller app. 
The app uploads all your contacts name, mobile number, photo and other details to its server. 
The app queries its server with the number which is calling you and gets persons name and displays it.
So you see Truecaller needs internet access to function whether through the Wifi / GSM/GPRS/EDGE etc. network. So you are in a location with only 2G and no Wifi, Truecaller cannot get a CallerID.