Current technology provides new challenges to improve skills on people with special necessities. In fact, persons with communications needs can take advantage of new devices and mobile applications to interact and communicate easily and in the most straightforward way. Augmentative and Alternative Communication (AAC) systems are oriented to users who cannot use natural language due to temporary or permanent impairments, providing an alternative to natural language for communication. In this paper, we present design guidelines for ECO (Easy Communication Application), an AAC tool designed with a User-Centered Design (UCD) approach to make functional communication easier for individuals with complex communication needs. ECO tool has two well-defined modules: the Communication User and the Communication Manager. The former is thought as an AAC tool, whilst the Communication Manager module provides a way to edit the content and fit the dynamics of use and application design to the user's interests. In addition, we present an evaluation of the ECO tool from two different perspectives. On the one hand, we have achieved a heuristic-based expert evaluation in order to analyse the functionality corresponding to the Communication Manager module of the tool, in order to check main accessibility requirements and improve functionality accordingly. On the other hand, we have evaluated both the Communication Manager and Communication User modules in a real context, carrying out a qualitative evaluation with final users and their therapists to check whether the tool is useful in concrete daily scenarios. Results corroborate research questions, demonstrating that ECO meets the most important accessibility requirements and it is highly valued by experts and end-users.