Investors have added nearly ¥1.3 trillion ($12 billion) to the combined market valuation of Japan’s two largest listed telecom companies since news first surfaced that Prime Minister Yoshihide Suga, who made reducing the country’s mobile phone bills a top priority, would step down.

Shares in Nippon Telegraph & Telephone Corp. (NTT) and KDDI Corp. continued to surge on Monday as traders digested the prospect of Suga’s potential replacements.

The prime minister’s narrowly focused policy platform had made reducing the country’s hefty mobile phone bills one of his key goals. Any successor is unlikely to be so passionate about the effort, which dates back to Suga’s time as chief cabinet secretary.

KDDI surged as much as 5.2% on Monday, the most since Nov. 2, while NTT added as much as 3.8%. The benchmark Topix Index added 1.1%, climbing to the highest level since 1990. The firms are Japan’s fourth and ninth largest by market value.

During Suga’s term, Japan’s carriers announced deeply discounted mobile plans, aiming to cut bills to "international levels.” The prime minister had long blasted the sizable profit margins at the three main carriers, and his campaign may have contributed to NTT buying out its unit NTT Docomo Inc. in a $40 billion (¥4.4 trillion) deal last year.

Hajime Sakai, the chief fund manager at Mito Securities Co., said that while investors were making such thematic buys since Suga announced he would step down, he cautioned that it remained difficult to narrow down specific bets, with no clear leader yet in the race to replace Suga.