This exposure assessment study introduces a cost-efficient add-on sensor attached to a smartphone to quantify auto-induced uplink RF-EMF transmission across 100–6000 MHz in multiple microenvironments. Activity-based surveys were conducted in seven European countries under non-user, maximum downlink, and maximum uplink scenarios. Reported power levels were lowest for non-user scenarios and higher during active use, with variation by country, urbanization, and setting. The authors frame the work as supporting future epidemiological research and planned validation against other tools.