Some 5-6 years ago there was very few actually if I think of it only Skype being the best VOIP service. But now there are lots, not just software applications which can be used on computers but also which can be downloaded onto your mobile phone.
As long as the other person has the same application the VOIP calls can be made free, and there are no charges the only thing that gets deducted is your data. These VOIP softwares applications not only are good for voice calls, but can also be used for video calls, chats, video sharing and photo sharing.
Most of these software applications are free to be downloaded on mobile phones. There may only be some OS who may charge for these applications but you will generally get these applications for free of cost.