Skype is the most popular service around for voice calls over the internet. You can also use live text chat and send files. Many international organisations rely on Skype to reduce their costs, using it instead of the telephone. It makes online collaboration between global grassroots communities easier, as it is both cheap and universal, supporting calls to landlines and mobile phones.
Despite its popularity, there are several security issues related to this software. While according to Skype, it encrypts both messages and voice calls, this only happens when both sides are using Skype. Skype does not encrypt calls to phone or text sent as SMS messages.
If both sides are using Skype, its encryption may make the call slightly more secure than an ordinary call over phone. But because Skype is a closed-source program, making an independent audit and evaluation of its claims about encryption impossible, it is not known how well Skype protects users and their information and communication. Governments are able to monitor conversations with, and even without, the co-operation of Skype.
Free Software, such as Jitsi for voice calls and Pidgin for instant messaginge, are recommended alternatives to Skype. They are more secure, but not always as user friendly.
If you decide to use Skype as a tool for sensitive communication, it is very important to take some precautions. Read the Notice about Skype's security in Security in-a-box: https://securityinabox.org/en/chapter_7_3
Talking and communicating with people around the world for free.
Very compatible across different operating systems and in different parts of the world
High number of users
Fears of a back door for government