Most Wifi is great for making calls, but if the signal is weak or the network is congested, you may get audio issues. If you do, hang up the call, and force the call over your mobile data network.

To change the network used for calls, 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.