Wireless carriers are still having a hard time keeping the iPhone 4S in stock, as Apple's new device continues to perform well against tough competition.

Customers at Verizon still have to wait more than three weeks for the 4S, while AT&T and Sprint are also in similar situations, according to the Wall Street Journal. Apple's supply chain is reportedly meeting all expectations, but higher-than-expected demand is leading to shortages.

AT&T, Verizon and Sprint are likely enjoying the surge in traffic the iPhone is bringing them, but Apple's inability to provide substantial stock of the device is also hindering the carriers from maximizing the full potential it could have on their businesses.

The incredibly high demand for the 4S is possibly more important for this version of its handset than any of its predecessors. Competition from Android is higher than it has ever been with devices like the Samsung Galaxy S2 and HTC Rezound posing real threats to sales, and the Galaxy Nexus, Google's new flagship device, is just around the corner.

Analysts have largely attributed Apple's past success with the iPhone to the fact that there were no devices formidable enough to challenge it, but the company is seeing its best sales of an iPhone ever while in the midst of its toughest competition yet.

Apple is falling behind Android in several statistical categories of late. HTC and Samsung both surpassed the company as the world's largest vendor of smartphones, and the number Android devices sold, and the amount of apps in the platform's app store are both quickly gaining on Apple's iOS.

Google's strategy of licensing Android to several manufacturers has allowed the platform to amass millions of customers quickly, but Apple's near-perfect fusion of its own hardware with iOS has been just as successful.

Apple is selling more handsets than ever before just as Google is hitting its stride with Android, indicating the two sides may be set to continue to battle it out for a long time to come.