Given that the carrier operates only 2G and 3G networks, the announcement further supports expectations that Apple is on the verge of introducing a new version of its handset built to run on so-called 3G networks.
SoftBank Mobile Corp, Japan's No. 3 wireless operator, recently beat all of its peers in annual net mobile subscriber growth for fiscal 2007, adding 2,676,500 subscribers thanks to its White Plan featuring lower fees.
As of April, SoftBank maintained a combined subscriber base of 18,779,100, which included 14,453,100 customers on its 3G network and 4,326,000 on its 2G network.
Talks between Apple and the carrier date back nearly two years to May of 2006 when rumors had the two companies collaborating on the then unreleased iPhone.
Apple had also been in talks with NTT DoCoMo, Japan's largest mobile wireless carrier, though an agreement between the two has not been announced.