Vodafone Idea shares tumbled as much as 24 per cent on Thursday, a day after the troubled telecom operator's board accepted Kumar Mangalam Birla's request to step down as non-executive chairman. Mr Birla had engineered the merger of Idea Cellular, which was part of his Aditya Birla Group, and the Indian operations of Vodafone in 2018 to form Vodafone Idea, creating what was the country's largest telecom operator at that time.