(Reuters) - SoftBank Corp (9984.T), Japan's No.3 mobile carrier, will cut its smartphone charges in Japan with a flat rate for unlimited voice calling and cheaper data plans, in a sign that price competition may be starting to pick up in Japan's mobile market.

The lower rates, to take effect next month, are similar to reductions this month by rival NTT DoCoMo Inc (9437.T), Japan's largest wireless provider. Regulators have grown increasingly critical of the three dominant carriers, blaming them for high smartphone fees and oligopolistic practices.

The authorities' complaints came at an awkward time for SoftBank CEO Masayoshi Son, as he tries to convince U.S. regulators that a proposed acquisition by his Sprint Corp (S.N), the No.3 U.S. mobile carrier, of No.4 T-Mobile US Inc (TMUS.N) would boost competition in the U.S. market, by giving the top two carriers more of a run for their money.

In addition to flat-fee unlimited voice calling, SoftBank will offer options allowing users to roll over unused data communications to the next month or to share them with family members.

It also will launch special offers or discounts for family plans, long-term users and subscribers aged 25 and under.

A voice-only smartphone service will be priced at 2,700 yen ($26.33) per month, while data plans will run from 3,500 yen per month for 2 gigabytes up to 22,500 yen for 30 GB.

Japan's wireless carriers had typically offered only high-end smartphone plans with minimum charges of 5,700 yen per month for 7 GB of data communications. The communications ministry has charged that the uniformly high fees showed there was insufficient competition in the industry.KDDI Corp (9433.T), the second-largest carrier, is also considering offering flat-rate, unlimited calling, sources familiar with the matter have said.

(Reporting by Yoshiyasu Shida and Teppei Kasai; Writing by Edmund Klamann; Editing byDavid Holmes)