ABUJA, Jan 5 (Reuters) - Shares of Airtel Africa hit an all-time high on Wednesday, lifting Nigeria"s main stock index, after the mobile phone company said it will use proceeds of a tower sale to reduce debt. Airtel , the third-biggest listed firm in Nigeria, with a dual listing in London (AAF.L), gained the maximum 10% allowed to 1,050.50 naira. The main index was up 1.05% at a seven-week high. Airtel Africa, with a presence in 14 African countries, announced the first closing of the transaction to sell its tower assets in Tanzania to a joint venture company for $176.1 million. Around $60 million from the proceeds will be used to invest in network and sales infrastructure in Tanzania with the balance used to reduce debt at group level.