The key feature of an intrinsically safe mobile phone is its ability to function safely without causing a spark or excessive heat, which could lead to an explosion. This is achieved through the design and construction of the phone's components, which are built to meet stringent safety standards, such as ATEX (Atmospheres Explosibles) and IECEx certifications. These phones are specifically engineered with low-power circuits, heat-resistant casings, and secure sealing to prevent the ingress of hazardous gases or dust.