Activist investor CtW Investment Group wants the Securities and Exchange Commission (SEC) to investigate two aspects of T-Mobile US’ accounting.

The operator did not adequately disclose a change in accounting estimates used to recognise revenue from its sales of Equipment Instalment Plans (EIPs), said CtW in a letter to the SEC.

This change had the effect of inflating GAAP-based earnings by $122 million, or about 23 per cent of net income, over the four quarters from Q4 2014 through Q3 2015, it said.

CtW also criticised the operator for generally give non-GAAP measures prominence over GAAP measures in its earnings releases, including a failure to provide the information required to reconcile reported non-GAAP Adjusted Ebitda to GAAP operating earnings, or net income.

In addition, the investor group is asking the SEC to investigate majority shareholder Deutsche Telekom for failing to rectify T-Mobile US’ nondisclosure of the change in its accounting estimates, which had an impact on the German company’s net income.

In May 2016, CtW wrote to Deutsche Telekom CEO (and T-Mobile US chairman) Timotheus Hoettges, asking him to review its findings and take appropriate action. The investor group said it has not received a response to its letter.

T-Mobile US’ recent strong performance, both in terms of revenue and profitability, has been a feature of the US mobile market. Its turnaround under the leadership of John Legere also had a positive impact on the financial results of Deutsche Telekom.

CtW is an activist shareholder who regularly urges changes in behaviour at large US companies. It holds a stake of less than one per cent in T-Mobile US.