A rebel investor in Vodafone Group Plc said on Tuesday it expected the mobile phone giant to address criticism of its 45-percent stake in U.S. joint venture Verizon Wireless over the next 12 months.

The investor, Efficient Capital Structure (ECS), earlier on Tuesday failed in an attempt to force Vodafone to hand over its stake in Verizon Wireless to shareholders by issuing so-called tracker shares.

But ECS Chairman Glenn Cooper told Reuters that shareholders expected action, and that Vodafone had simply bought some time by promising last week to maximize the value of its stake in Verizon Wireless.

This wasn't about the votes but about action from the company, he said in an interview on the sidelines of Vodafone's annual shareholder meeting. Now we're convinced that Vodafone will take action about Verizon (Wireless).

I'd be very surprised if we have the same debate the same time next year, he said. If within that timeframe there is no action, the board will face an investor revolt and an EGM (extraordinary general meeting).