What's the best app to get free phone number? How about WhatsCall?

That's by Cheetah Mobile, and so very likely it will be monetized by lots of ads and by data mining your stuff and selling it to third-parties.

There's Google Voice, which has been recommended and discussed on here from time to time.
https://voice.google.com/u/0/about

 

Google Voice is only available in the US. It works really well but uses your cell number/service to place/receive calls by default, it's really a call forwarding service. You can use it on wifi without a SIM but it has to be tinkered with to set up correctly, it would need the Hangouts Dialer too, (more tinkering). In order to set it up you'd need a real US phone number (you can use someone else's though). Even then only calls to the US and Canada are free so any international calls would cost you (but prices are reasonable I think), and I'm not even sure if you'd even be able to place a call while in Canada.
(I've tried it on wifi and it does work with the HO's dialer)