Sprint (NYSE: S) has made impressive progress in cutting costs and increasing liquidity as it continues to navigate difficult financial waters, analysts at UBS wrote this week. But serious questions remain about whether the operator can continue to add subscribers, trim its budget and maintain its network all at the same time.

The country's fourth-biggest mobile network operator posted decidedly mixed results for its fiscal fourth quarter two weeks ago, adding 22,000 postpaid phone net adds but suffering a net loss of $554 million. But its $310 million in operating income for the fiscal year was positive for the first time in nine years, and it ended the year with 1.2 million postpaid net additions, 438,000 of which were phones.

"Sprint put up solid numbers for F4Q15," UBS analysts wrote in a research note. "With its recent Network LeaseCo and Mobile Leasing Solutions transactions and new $2B bridge facility, Sprint has largely put to rest near- to medium-term concerns over liquidity. In addition, the company appears close to an inflection in service revenues, which has generally been good for wireless stocks."

And the operator is "making solid progress" in its efforts to cut $2 billion from its annual budget, UBS said. It has already attained $250 million in savings per quarter, UBS said, and may reach its goal of saving $500 million per quarter by the end of the year.

The moves are likely to help Sprint weather a coming financial storm. Sprint owes $10 billion that will come due by the end of 2020 and has to make $2.3 billion in debt payments this year.

But those savings may cost the carrier subscribers in the coming months. Decreased marketing budgets may lead to slower customer gains, and its lowered capex guidance has investors concerned Sprint's network won't be able to keep pace with its three larger rivals.

"For us, the issue with Sprint remains the uncertainty on the company's ability to grow the sub base and return to revenue growth on a sustainable basis while also aggressively cutting costs (including marketing, making it harder to attract gross adds,)" according to UBS. "Likewise, the capex slowdown could make continued churn gains more difficult, as network performance improvement may stall."

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