Fixed mobile substitution, usually abbreviated to FMS, is the use of a mobile phone (cellular phone) instead of a fixed, wired, POTS telephone.
Although strictly, this could simply be as the result of a subscriber making an individual choice (for example, "25% of the population of Finland do not have fixed phones, but use mobiles all the time, even at home"), the term is usually used to describe a deliberate tactic of a carrier to promote such a behavior.[citation needed]
One such technique is the use of "homezones" (where calls on a designated cell, covering the subscriber's home, are at a preferential tariff, or even free). Although very popular with subscribers, and delivering good uptake, these are expensive for carriers, and outside of a few specific countries (e.g. Germany with Genion and Zu Hause services) they are not common.
The use of femtocells promises to be a more attractive (and potentially more profitable) approach.[citation needed]
The term FMS is in contrast to fixed mobile convergence (FMC), which is usually used to describe the use of two different technologies (hence "convergence") to deliver a similar user experience. Examples include the use of dual-mode handsets with both cellular and WiFi technologies and hand-off between the two.