There seem to be apps for recording for local or international calls but none for Internet calls.
Some apps on the Google Play Store claim to be able to record calls on Viber or Skype. I even found some apps not in the play store. I tried some, but I'm not sure if they work. I haven't gotten around to testing any yet. Some reviews of the apps I found seem to be mixed. I don't feel like reinventing the wheel.
I have a second phone that I would use except that sometimes the calls I make require earphones.
If it matters, the phone I use for Viber, Skype, etc calls is a Samsung Galaxy S6 Edge Plus, and my other phone is a Samsung Galaxy Mega.
Please don't bring up any stuff about legality.