A Voice application is a piece of software which when loaded on your multimedia enabled PC, Smartphone or Tablet allows you to make VoIP calls over a network connection. The motivation for using a Voice app on a mobile telephone is simply with regards cost. It is far cheaper to make a call over a WLAN/Wi-Fi connection than it is to use the service provider’s mobile data plan.
In order to make use of the voice application, you will probably have to register with a SIP service provider in order to get a SIP address and routing service. That may well be the only costs involved as most voice apps come free or with free versions. The paid versions are usually aimed at businesses with specific quality, reliability and availability demands so if that will be the predominant use then paying for the software is probably a prudent move. For most home use though the free versions will provide all the features and quality required.
There are different types of voice apps available so you need to get the one specific for the device you are using. For example, download and install the version that matches your OS version when using a PC or laptop. On a mobile device check to get the mobile version for not just the OS whether that be IOS or Android – but for the actual model of phone or tablet as they vary a lot. Some voice apps like Google have the softphone application embedded in the web browser. Also, many softphone functionality is coming packaged into cloud based applications.
There are some drawbacks with using voice apps the main concern was to receive calls the PC needed to remain on but that is not such an issue when being deployed onto a smartphone or tablet.