By Richard Martin

Responding to the Japanese governments call for fully open networks and interchangeable devices, NTT DoCoMo, Japans largest wireless carrier, said it will begin distributing SIM-free phones next spring. Starting in April 2011, NTT DoCoMo will remove carrier-specific locks, called SIM locks, from all of its mobile phone models, the company said in a statement.

DoCoMo CEO Ryuji Yamada told the Nikkei daily that users will be able to keep their handsets and switch to a different service provider simply by replacing the Subscriber Identity Module (SIM) card. NTT DoCoMo is one of the first major wireless carriers to begin selling unlocked devices.

Last month Japans Ministry of Internal Affairs and Communications called on all wireless carriers to voluntarily unlock their handsets to give customers more freedom of choice. One of the most advanced mobile-phone markets in the world, Japan has more than 100 million mobile subscribers, in a country of 127 million. With around 55 million subscribers, DoCoMo is the countrys largest mobile operator.

It also boasts the nations top wireless network, and is said to be banking on the superiority of its service to offset any losses from defecting customers in the move to unlocked phones. Softbank Mobile Corp., the exclusive carrier for the iPhone in Japan, has not said whether it will unlock its phones.

Nokia, the worlds largest handset manufacturer, has for many years sold its devices unlocked. Google attempted to bypass the major carriers by selling the Nexus smartphone direct to users, but that effort largely failed.

Last month NTT DoCoMo launched a trial of its 4G LTE network, and last week it began receiving shipments of LTE base station equipment from Japanese vendor NEC Corp.