TalkTalk said it expects its full year earnings to be at the lower end of its guidance range after it reported a first-half loss.
The telecom booked a £71m net loss, compared to a £21m profit a year earlier.
Revenue fell 5.1% to £856m.
The company grew customer numbers by 46,000 as it prioritized volume growth.
"We see continuing strong demand for our competitively priced propositions for consumers and businesses and an opportunity to step up H2 investment in driving materially higher base growth, continued re-contracting activity, and further fibre take-up," the company said.
"As a result, we expect FY18 headline EBITDA to be towards the lower end of our guidance range, and will enter FY19 with a materially higher quality, lower churning and larger customer base."