Mobile radio telephone (or
0G) systems preceded modern cellular mobile telephony technology. Since they were the predecessors of the first generation of cellular telephones, these systems are sometimes retroactively referred to as
pre cellular (or sometimes zero generation) systems. Technologies used in pre cellular systems included the
Push to Talk (PTT or manual),
Mobile Telephone System (MTS),
Improved Mobile Telephone Service (IMTS), and
Advanced Mobile Telephone System (AMTS) systems. These early mobile telephone systems can be distinguished from earlier closed
radiotelephone systems in that they were available as a commercial service that was part of the
public switched telephone network, with their own telephone numbers, rather than part of a closed network such as a police radio or taxi dispatch system.