Image source: The Motley Fool.

What: Shares of telecom company Sprint (NYSE:S) increased as much as 25% on Monday after it reported solid first-quarter results, including revenue exceeding analyst expectations and the company's highest first-quarter postpaid phone net additions in nine years. As of 12:50 p.m. EDT, shares are up 24.5%.

So what: For its first quarter, Sprint reported revenue of $8.01 billion and an earnings-per-share loss of $0.08 per share. The loss was in line with expectations, but revenue was ahead of analyst expectations for revenue of $7.99 billion. 

Sprint's slight outperformance on revenue compared to analysts' expectations. However, it probably isn't the main reason for the stock's big move on Monday. Instead, investor optimism for the stock is likely driven by Sprint's positive update on customer trends. In addition to reporting its highest first-quarter postpaid phone net additions in nine years, it reported its "lowest postpaid phone churn in company history," at 1.39%. Further, Sprint CEO Marcelo Claure said the company was "finally ... postpaid net port positive against all three national carriers after five years."

Now what: The company's positive key customer metrics, along with Sprint's achievement of increasing wireless net operating revenues by 1% year over year, offers investors positive signs amid its turnaround effort.

Going forward, Sprint management is guiding for between $1 billion and $1.5 billion in operating income in fiscal 2016 -- a huge improvement from its $310 million in operating income in the trailing twelve months.