Complex sentence: Until around 1930 the only way the controls could contact the mobile patrolmen was by telephone, so they waited by public telephone boxes for the callout, later they were equipped with basic early radio sets for two way contact with their local headquarters.
Split into simpler sentences: Until around 1930 control could only contact the mobile patrolmen by telephone, so they waited by public telephone boxes for the callout. Later they were equipped with radio sets for two way contact with their local headquarters.