How do you stop regular Voice Calls from killing WiFi calls?

[CN5555]

I have a Galaxy S4 (I know it's old, but I like it because I can change the battery and it's helpful to have multiple while travelling). However, when using apps for VOIP calls like LINE, the LINE call will get killed when a regular call comes in whether I answer it or not. Is there a way to keep regular calls from interrupting my VOIP calls?