User activity monitoring has a considerable potential for a whole range of new applications in different environments: user surveillance, military purposes, health-care, etc. More precisely, in health-care there is an implicit potential application monitoring elderly people or patients suffering an illness. Besides, nowadays our society is more than ever exposed to an aging population, which is one of the main concerns of present administrations. Any technical approach alleviating the ever increasing health-care budget will be welcomed. Thankfully, recent progress in communication technologies and sensor miniaturization has opened the door to new approaches overcoming the above mentioned situation, involving, for example, remote supervision of home-confined elderly people. In particular, a real-time monitoring system of human activities would be an interesting solution to supervise aged people or people deserving medical attention. In the present paper we expose the development of a real-time monitoring system of human activities using a single cell phone equipped with some sensors. This way the monitoring system is reduced to the minimum expression, no especial hardware is required, as far as a conventional cell phone will monitor the user, classify the user activities with respect to a customized rule’s set and finally trigger the corresponding alerts when necessary