In recent years the mobile-only population has gained more and more importance for survey research since it has increased consistently in most industrialized countries. In some European countries like Finland, Sweden or the Czech Republic mobile phone penetration rates have exceeded 90 percent; at the same time landline penetration rates have declined. Thus, mobile-only rates have surpassed 30 percent in many countries. As known from the literature (e.g. Blumberg and Luke 2010; Fuchs, 2008; Ehlen and Ehlen, 2007) mobile-onlys differ from the landline population as they are young, single, male and either belonging to a low-income or a high-income segmentation. Consequently it is necessary to complement traditional landline telephone survey methods with mobile phone survey in order to cover the mobile-onlys in telephone surveys. So far, survey researcher assume that individuals who give up their landline telephone service remain mobile-only for a longer period of time (e.g. until they settle down and build a family). In this paper we will test the assumption according to which the decision to become mobile-only is a permanent status. Are individuals moving in and out of the mobile-only population more frequently? And if so, what are the circumstances conveying a change in the telephone equipment? In our paper we will present results from a mobile phone panel survey conducted in Germany from spring 2009 to spring 2011 (N=1400). In each of the four panel waves we asked our respondents for their mobile and landline telephone equipment. Thus, we are able to track changes over a period of approximately two years. Results indicate that far more individuals than expected switch back and forth between landline and mobile-only status. Some respondents exhibit even multiple changes over the course of one year. We will report changes in telephone service status and the socio-demographic specifics of this group.