If your PC has several network adapters, including real physical and virtual, Freephoneline at startup offers to choose the host (a network adapter with IP address) to use during calls. This feature allows to channel phone calls via a different adapter compare to the rest of internet traffic on that PC, thus contributing to call audio quality. Unfortunately, this feature doesn't work correctly in Win 10 64-bit. No matter what local physical IP address (adapter) you select, the calls will be channeled by Windows 10 through the network adapter that has lower Metric value set in its IPv4 Advanced Properties. Manually changing that value doesn't help, since its global, meaning it will apply to all traffic on that PC, and not only Freephoneline calls.
Is it possible to update the app to comply with Win 10 networking? Alternatively, what method would you suggest to reliably bind Freephoneline app to a certain network adapter installed on one's PC? Does this method work for you in your own tests? You can check, what network card the traffic is channeled through with Nirsoft AdapterWatch.