The Zensens' call to missionary service occurred during a missions conference in 1971 at their home church, where TWR founder Dr. Paul Freed presented the needs of TWR. In 1973, they moved to Bonaire, where George was a transmitter engineer and Joan filled a variety of roles.
In 1980, the Zensens transferred to the TWR Guam station, where George served as chief engineer. Joan again served in a number of capacities, including responding to listener letters. George and Joan relocated to the HCJB Engineering Center in 1997 to help build HC100 high-power shortwave transmitters. They continue to serve there as retirees.