Many smartphone users are no doubt looking forward to next month, when Japan’s telecom giants will launch new monthly plans that are considerably cheaper than current deals.

But for the wider industry, it will be the dawn of a new price war.

With the three megacarriers — NTT Docomo Inc., KDDI Corp. and SoftBank Corp. — making bold new offers, smaller rivals known as mobile virtual network operators (MVNOs) are likely to face an uphill battle.