Cryptocurrency transactions are irreversible. If you send cryptocurrency to a third party, you cannot reverse it or stop payment. When you send cryptocurrency to a blockchain address, you must be certain of the legitimacy of any involved third-party services and merchants, and only send cryptocurrency to entities you trust.
Pro tip: If it sounds too good to be true, it probably is.
Phishing sites are malicious websites which mimic an authentic site in order to trick visitors into entering their login credentials or other sensitive information. These fraudulent websites are distributed through a variety of methods including email, SMS text messages, social media, and search-engine advertisements.
One of the best ways to avoid phishing sites is to always make sure you're accessing https://www.coinbase.com directly. Phishers will often use URLs like www.c01nbase.com to conduct these scams. Detailed information on phishing can be found in this help article.
Report phishing to your mobile provider
If you use a US-based cell phone carrier like AT&T, Verizon, Sprint, or T-Mobile, you can help reduce phishing attacks by sending the contents of suspicious SMS messages to 7726 (SPAM). This service is free, and allows mobile carriers to detect and block malicious messages on their network.