SMS blockers are a clever evolution of the old ransomware or scareware tactic of demanding a payment in exchange for removing some malware on a PC or decrypting the user s hard drive. These tactics have been extremely profitable for scammers in the last four or five years, earning some gangs millions of dollars per year. In order to disinfect the PC, the victim must send an SMS message from her mobile phone to a premium number controlled by the attacker, typically at a cost of about $10. The new scam relies on an ecosystem of entities behind the scenes in order to work.
Source: https://threatpost.com/sms-blockers-new-face-ransomware-060210/74043/