In my opinion, today most people use Skype or Viber. But to not install all software in a row, let's think which one to choose.
Skype, perhaps, can be considered the most popular and oldest among the messengers installed in PC and mobile gadgets. Useful this Skype version is for desktop PCs, laptops, netbooks, tablets and smartphones based on Android, iOS. Well, in order to connect with the right person, you must know his username in this system, it is necessary that the subscriber was registered in Skype. Using Skype you can call a regular phone number, but such calls will be paid and the amount of the fee will depend on the specific country.
Viber came later, but quickly gained popularity. From this, there are also versions for PCs and mobile devices. As login you will use the phone number that must be indicated when registering. And here lies the catch - Viber installed on a smartphone will automatically check your contact list and notify everyone, which he also installed, you to join their warm company. And even if you put Viber on computer, anyone who has your phone number, learn about installing this app you. By the way, calls with Viber on normal phones it is also possible, and will also be paid.
So, what conclusion can I draw from the above? Skype and Viber, in General, have similar capabilities - you can make free calls and send messages to worldwide users of the instant messenger that you choose, so it will stay on the program, which is more common among your colleagues or friends.