Our work aims to instrument exchanges between learners during mobile learning activities. To this extent, we propose the use of digital annotations associated with physical objects involved in the learning situation. We introduce the CALM model (Contextualized Annotation for Learning through Mobility), which allows contextualizing annotations with information about environment and learner's situation. This contextualization should facilitate annotations reuse by other learners in other contexts. We present an application of the CALM model in the context of museum visits.