Earth represents electrical ground and magnet represents the electromagnet used to generate tone.
Usually the PBX is the trunk circuit side and the telco, CO, channel-bank, or Cisco voice enabled platform is the signaling unit side. I - This is the most common interface in North America. This is the most common interface type used outside of North America. These implementations describe the number of wires used to transmit audio signals. Wink Start - Wink is the most commonly used protocol.