Lost iPhone 4S - phone is on and Find my iPhone should be enabled, but says it is "offline"

I lost my iPhone 4S. Find my iPhone says that it is offline, but when I call my phone, it rings before going to voicemail, meaning that it is on. I have found this iPhone before using Find my iPhone, so it was definitely turned on. It is probably not connected to wifi but should have 4g. Any suggestions?