Although many researchers have examined knowledge building in traditional settings and distance learning, few have examined middle school students' building of mathematical knowledge using mobile phones. The present study uses two well-known models of knowledge building to carry out the examination: the interactive analysis model of knowledge building phases developed by Gunawardena, Lowe and Anderson (1997) and the six themes model of knowledge building characteristics developed by Scadamalia and Bereiter (2006). The findings show that the middle school students participating in this research went through all the knowledge building phases suggested by Gunawardena, Lowe and Anderson (1997). They further experienced other knowledge building phases that fit the authentic context in which they learned. Participants advanced their knowledge of ideas as a community, collaborating to carry out authentic activities using mobile phones. They demonstrated constructive and critical use of information in general and of authoritative information in particular. Participants worked as mathematicians, especially during the second part of the experiment, when they suggested real world phenomena to explore using the mobile phone. My conclusion suggests learning mathematics by carrying out authentic activities using mobile phones, to encourage and enrich the mathematics knowledge building of students in K-12. (Contains 1 figure.)