In other countries they are known as single wire telephones (SWT) and also as earth-return telephones.
Later designs for SWTs were published by Nigel Lovell (`Practical Earth-return Telephone Design', CREG Journal, 13, Sept. 1993 pp.9-12) and by K. Walraven (`Single Wire Communications', Elektor Electronics, Nov. 1994 ).
The NSW Cave Rescue Squad, based in NSW, Australia has used Michiephones for cave rescue communications since 1974. They are still the backbone of the CRS's in-cave communications system.