The prevalence of Skype often means that other VoIP software is forgotten about. One alternative is TeamSpeak which is available for a number of platforms - both desktop and mobile - ranging from Mac, Windows and Linux to Android and iOS.
For the most part, TeamSpeak works precisely as you would expect, making it possible to maintain a list of contacts and receive notifications of when they are online to chat with. These contacts can be divided up into categories depending on the network they are associated with - such as business or personal contacts - and it can also be used to chat with players of a number of online games. In addition to enabling you to conduct voice chats, the app can also be used to send text messages.
TeamSpeak supports Push To Talk, but is can also be switched to voice activation mode for hands-free operation. There are a large number of public servers available that can be used to connect to other users, but there is also the possibility of renting a server of your own if you require it for work.
The app enables you to be connected to multiple servers at the same time making it possible to chat with people from different networks at the same time, and status notification mean that t is easy to keep track of who is online and who has marked themselves as being 'away' for a while. TeamSpeak is a solid online chat platform, it is just a bit of a shame that the Android version is free while the iOS edition is a paid-for app; this may change in future versions to create a more level playing field.