Most Wifi is great for making calls, but if the signal is weak or the network is congested, you may get audio issues. If your wifi network isn't correctly configured, you might even find that your line can't connect - You'll get a red oval at the top left of the keypad screen. To overcome this issue, just use your mobile data for the calls.
To change the network used for calls, open the app and use the cog on the keypad screen, choose 'Preferences' and then 'Network'. The top item is 'Network Constraint' tap this, and you can choose the default which is 'Any Network'. Or, if the Wifi is bad, choose 'Mobile Data' to force the call to use your mobile data allowance. Be aware that there may be charges or limits from your operator for this. To save just use the top left to go back through the screens. Dial your call again and see if you get a better connection.