NEW DELHI, July 10 (Reuters) - Top Indian mobile phone carrier Bharti Airtel Ltd said on Monday it would invest 20 billion rupees ($310 million) over three years to improve its services. As part of the plan, the company will invest in redesigning its 2,500 stores across the country, revamping its app, and improving its postpaid services, said Gopal Vittal, Bharti Airtel's chief executive for India and South Asia, at a news conference. India's highly-competitive telecoms sector has been disrupted by cut-price offerings from new entrant Reliance Jio Infocomm Ltd, backed by the country's richest man Mukesh Ambani. Bharti Airtel and rivals including Vodafone Group Plc's Indian unit have cut prices to retain customers. Bharti Airtel will allow customers to carry forward up to 200 GB of unused data a month to the next cycle, Vittal said. ($1 = 64.5425 Indian rupees)