The use of reactive detection technologies such as passive and active sensors for avoiding car accidents involving pedestrians and other Vulnerable Road Users (VRU) is one of the cornerstones of Cooperative, Connected, and Automated Mobility (CCAM). However, CCAM systems are not yet present in all roads at all times. The use of currently available technologies that are embedded in smartphones, such as location services and Internet access, are enablers for the early detection of VRUs. This paper presents the proof-of-concept of a system that provides vehicles with enough information about the presence of VRUs by using public cellular networks, an MQTT broker, and IEEE 802.11p-enabled hardware (a roadside unit and an on-board unit). The system was tested in an urban environment and in a test track, where its feasibility was evaluated. Results were satisfactory, proving the system is reliable enough to alert of the sudden appearance of a VRU in time for the driver to react.