Sprint Nextel has paid off a $1 billion loan under its $4.5 billion revolving credit facility. The company is currently about $21 billion in debt, according to its third-quarter financial statements.

The payoff comes just days after the S&P warned that the carrier’s junk-level credit ratings were on watch for downgrade. Sprint’s credit ratings are already two notches into junk at BB. The carrier has faced ongoing losses and customer defections despite improvements to both its network and customer service.

Sprint CEO Dan Hesse says the company’s customer losses will stabilize, but his promises have done little to reassure investors. Sprint Nextel stock has fallen almost 35 percent since mid-May and is currently trading at $3.43.

Not everyone is as pessimistic as Standards and Poor’s, however. Credit Suisse recently upgraded Sprint’s stock on predictions that the carrier would stop losing customers in its fourth quarter. In a note to investors, Credit Suisse analyst Jonathan Chaplin upgraded Sprint to “Outperform” from “Neutral” and raised his share price target to $6 from $4.

Aside from the upgrade and debt payoff, Sprint also announced it began reselling Clearwire’s 4G service in San Antonio and Austin, Texas. Sprint and other investors recently pumped an additional $1.56 billion into Clearwire as part of the mobile WiMAX provider’s build-out efforts.