I am with Vodafone, and they have removed VOIP from the S7. Don't want to use a 3rd party app, only the one that is on Android as standard. Will have to root my phone, and use the dialer built into Mrshmallow/Nougat.
Maybe Vodafone will listen to their customers and put it back in the Nougat release.
Fortunately for me 02 haven't removed the VOIP facility from my S7 edge, however you could use Facebook Messenger to make phone calls. Though they would have to be friends of yours.
There are plenty of VOIP apps on the Google Playstore, however I can't make any recommendations.
Hopefully someone will provide a more helpful reply.