UScellular has asked the Federal Communications Commission (FCC) to approve T-Mobile's $4.4 billion acquisition for most of its business.

The regional cell provider wrote a letter to the FCC warning that the proposed deal is essential for the company's long-term future.

In May, T-Mobile announced it had agreed to scoop up UScellular's operations, including its customer base and around 30 percent of the company's wireless spectrum.

UScellular will retain ownership of its other spectrum plus its towers, with T-Mobile entering into a long-term arrangement to lease space on at least 2,100 additional towers being retained.

In its letter to the FCC, UScellular, which serves more than four million mobiles across 21 US states in mostly rural areas, said it expects to continue losing subscribers should a deal not be allowed.

The telco noted in its letter that it's the seventh-largest wireless provider in the US, accounting for around one percent of connections in the country, making it difficult for the company to challenge the three big American carriers; AT&T, Verizon, and T-Mobile.

It said it has "consistently lost subscribers in recent years," while the market has ramped up in competitiveness from both traditional wireless providers and cable companies.

"That intensity has, in turn, led to aggressive pricing and promotions—and further challenged UScellular’s subscriber numbers and financials," said UScellular, which also bemoaned the cost of 5G, noting that it took on significant debt to purchase the mid-band spectrum needed to compete in that market.

According to the company, it's also lost subscribers as it's had to scale back its investment into 5G as it looks to ease debts.

"While UScellular has been pulling back on its network investments, its competitors have been spending more to expand their networks and enhance their network quality and customer experience in UScellular’s footprint. UScellular has fallen behind its competitors and the gap is continuing to grow."

Attempting to make their case, UScellular said that its deal with T-Mobile will benefit customers of both businesses "due to the complementary nature of the spectrum and wireless operations of each provider."

The carrier said that the planned acquisition will lead to a better network experience for subscribers as there will be access to greater capacity and faster speeds, while T-Mobile will be able to quickly deploy UScellular spectrum once the deal closes.

The transaction is expected to close next year but has not been welcomed by all. In July, a group of Democratic senators, including Bernie Sanders asked the Justice Department and the FCC to scrutinize the deal.

“Additional consolidation in the market would have far-reaching effects, reducing choices for consumers, further concentrating wireless spectrum holdings, and potentially leading to higher prices and other harms for consumers across the country,” the senators wrote in the letter, as reported by CNBC at the time.