A radiotelephone (or radiophone) is a communications system for transmission of speech over radio. Radiotelephone systems are not necessarily interconnected with the public "land line" telephone network. "Radiotelephony" means transmission of sound (audio) by radio, in contrast to radiotelegraphy (transmission of telegraph signals) or video transmission. Where a two-way radio system is arranged for speaking and listening at a mobile station, and where it can be interconnected to the public switched telephone system, the system can provide mobile telephone service.