If you need GPS to locate the techs, you need to check the management. Techs who don't answer phone or radio should be brought in for conference and told how it will work. We had that issue at another place I was employed and I told my boss the same thing. It's a management issue. Disciplinary action will make one answer their phone. And before anyone jumps, yes, I am a tech, and have been sent many miles out of the way because someone wouldn't answer their phone( during business hours) causing customer to pay extremely large travel times. Finally convinced the bosses that each tech should be responsible for a specific area and learn the lifts in their area. There are exceptions but they are few.