Shhh... hear that sound? That's the clock ticking off the seconds on Clearwire's (Nasdaq: CLWR) future. The company has until 5 p.m. today to pay the $237 million it owes in interest on its $4 billion of debt.

Wait a second, the ticking just stopped! Clearwire made the payment. We can all breathe a sigh of relief. End of story... or is it? Why were we all waiting in suspense anyway?

The plot
Clearwire CEO Erik Prusch shook up the telecom world two weeks ago when he told The Wall Street Journal the company was considering not making that payment. "It would be a significant drain of our cash, so we have to evaluate everything in terms of our decision of where we're going," he said.

But Clearwire did have the money to make that payment -- at least according to its latest SEC filings -- almost $700 million in cash and cash equivalents. So the big question is, why threaten to miss a critical deadline like this?

One theory making the rounds is that this was a game of chicken that Clearwire was playing with its main wholesale networking customer (a substantial owner of Clearwire's equity), Sprint Nextel (NYSE: S). Clearwire needed a contract extension with Sprint that goes beyond 2012 just to stay alive. One analyst, Ping Zhao of CreditSights, told Bloomberg that Clearwire may have been threatening default to pressure Sprint into a new agreement.

Best friends forever?
Sprint was the first wireless carrier to claim 4G capabilities and has been using Clearwire as its 4G WiMAX network provider. The problem is WiMAX is a much slower technology than LTE, which Verizon (NYSE: VZ) and AT&T (NYSE: T) have already deployed in their 4G networks. Clearwire has said it needs almost $1 billion to upgrade its network to LTE specs and to maintain its current equipment. But Sprint has seemed loath to bail out Clearwire even as they seem so dependent on each other.

Yes!
If it was a ploy, it seems to have worked. Sprint and Clearwire announced today agreements that will make it possible for Clearwire to go ahead on an LTE buildout. The new deals will provide Clearwire with over $900 million, with two-thirds of that paid in 2012, for continued WiMAX network service through 2013. Total funding will reach $1.6 billion.

And so Clearwire then paid the interest.

What if?
But what would have happened if it hadn't?

If Clearwire didn't get that check to the debt trustees' office by 5 o'clock, the world wouldn't end -- at least not right away. There is still a 30-day grace period to pay it off, but Standard & Poor's said in a statement that if Clearwire didn't pay up within five days, it would drop the company's rating to the bottom of the barrel with a D (for default) rating. S&P has already downgraded it to CCC-.

Crisis averted?
That disaster seems to have been avoided for now, but with these two companies, one can take nothing for granted. As analyst Ping Zhao pointed out, "Usually what's good for Clearwire is bad for Sprint."

In the past I have given Clearwire an underperform rating in CAPS. This news doesn't change that, but I will admit that it does bring some hope.

Keep an eye on Clearwire and Sprint by adding them to the Fool's My Watchlist: