E&M (Earth and Magneto) signalling is typically used on a connection to a PBX, rather than signal and voice traversing the same wire, E&M uses seperate paths for each.

E&M call setup:
  • Calling PBX actives it's M lead
  • Called PBX detects request on it's E lead
  • Called PBX enables it's dial register (collect digits)
  • Calling PBX sends dialled digits
  • Called PBX actives M lead to advise call is answered
Five types of E&M signalling, Cisco support 1,2,3,5.
E&M supervisory signalling types must be identical on both devices (supported on all E&M types above):
  • Wink start (calling PBX activates M lead for short time, called PBX then knows to collect digits)
  • Delay start (calling PBX activates M lead, called PBX actives M lead then collects digits)
  • Immediate start (calling PBX actives M lead and waits for specific time, then blindly sends digits)