Industry observers have attributed the relatively low mobile phone penetration rate in the United States, compared to that of Europe, to the subscriber-pays model. In this model the convenience of the mobility is charged to the subscriber. Callers from outside the local-calling region of the assigned number, however, pay for a long-distance call, although domestic long-distance rates are generally lower than the rates in caller-pays systems. Conversely, an advantage of caller-pays is the relative absence of telemarketing and nuisance calls to mobile numbers. The integrated numbering plan also enables local number portability between fixed and wireless services within a region, allowing users to switch to mobile service while keeping their telephone number.