How do I set a default application that gets used without enquiry?
While it cannot switch the default app for something, Tasker can still do this. 
Llama may be able to do it also, but you'll have to set it up like this: 

Set the Stock Dialer as default. 

Make a tasker profile that enables when you are connected to WiFi and triggers when you open the dialer-- the triggered action could then be to open your WiFi calling app. 

I hope that works--