If your phone has been bombarded by unsolicited text messages promising free Walmart gift cards or an iPhone for completing a survey, you're not alone. Indeed, spammers have "infiltrated the last refuge of spam-free communication," the cellphone, with frustrating success, says Nicole Perlroth at the New York Times. And the scourge is only getting worse. Here, a look at the annoying rise of mobile spam, and what you can do to protect yourself:

How bad is the problem?
Last year in the United States, 4.5 billion spam texts were beamed to people's phones. In 2010, that number was only 2.2 billion, according to Ferris Research. Computers "generate millions of possible number combinations" that don't necessarily have to be working numbers, says Perlroth, and the problem is magnified because spammers can mass text across multiple service providers. Text-message marketing is still less commonplace than email spam, but it's a "growing menace," says Perlroth, and while it may seem "harmless, if annoying," there is "potential for significant damage."

Why is this spam so harmful?
First off, if you don't have an unlimited text messaging plan, you can be charged for each spam text you receive. And spammers' motives are often nefarious, says Amy Kraft at SmartPlanet. Their surveys may ask you for seemingly harmless information such as your salary or favorite vacation spots, which spammers then sell to digital marketers. Another type of spam signs recipients up for a never-ending stream of texts if they press the wrong key. And the process of unsubscribing from lists can be aggravating  — replying with "NO" or "STOP" may simply verify to spammers that the phone number works. 

Isn't mobile spam illegal?
It sure is. Consumers should be protected under two federal laws: The 2003 Can Spam Act, and the Telephone Consumer Protection Act, which umbrellas 2003's Do Not Call Registry. Actual enforcement is another issue, though. Spammers are incredibly difficult to track down. Once a phone carrier blocks a spammer's number, the spammer can simply switch to another to blast thousands of unsuspecting phone users from automated lists. 

Is anything being done?
The mobile industry has acknowledged the growing problem, and is taking steps to rectify it, for instance, by joining ranks with anti-spam software company Cloudmaker. Phone owners are strongly urged to report spam numbers on the websites of all major wireless carriers — AT&T, Sprint, T-Mobile, Bell Mobility, and Verizon Wireless. And if you own an Android phone, there are apps you can download to enhance spam filtering. 

Is there any upside?
Well, it could be worse. The United States' spam woes are small compared to China's. Last year, an estimated 100 billion spam messages were sent to the country's one billion mobile phone users.

Sources: New York Times, SmartPlanet, TechGadgetsWeb, Times of India