The Vodafone Group, which recently acquired control of mobile operator Hutch-Essar for $10.9 billion, has received a call from an influential activist investor lobby to consider a restructuring proposal that could unlock up to $75 billion for its investors.While acknowledging the receipt of the letter from Efficient Capital Structures (ECS) Vodafone said in a regulatory filing today that it is reviewing the cotents of the letter and would make an announcement in due course.ECS is asking Vodafone to submit a number of resolutions at the company's annual general meeting on July 24 concerning the potential restructuring options for the company."We believe Vodafone is undervalued because it has an inefficient capital structure. Its most valuable asset is a passive, minority share and its balance sheet is under-geared," ECS said in a separate statement.The resolutions are designed to put this right and, if implemented, could release between 17-38 billion pounds of value to shareholders, Glenn Cooper, chairman, ECS said.The fund has also established a website Vote4Value.com to garner support of Vodafone shareholders for its proposals.