According to me both income tax and GST is applicable. Here the company is selling the phone at a price and all the risk and reward is transferred by the company. The company issues an Invoice for the same. It forms part of Turnover. The company further will buy back the phone after the completion of the specified period as per the terms of sale. At the time of buyback, the expenses shall be debited to p&l account.
Further in terms of GST, the company collects GST for sale of phones and then at the time of buyback, it is pays tax under RCM 9(3) and claims ITC in subsequent month.