If someone calls an iPhone with their Millitalk app is it free to receive that call?

My boyfriend and I are in a long distance relationship. He has Wi-Fi and I don't so Skype was fine for him but it has used up all my data. I can't call him on my phone because he doesn't have a canada wide plan, but I do on the otherhand. If he gets a calling app such as Millitalk or textme on his phone and uses that with his Wi-Fi, will it be free to call him from my regular phone to that app? Or if he calls me off that app to my phone? Also if you know of any other good apps that would be very helpful too.

Well its not really calling, its just over Wi-Fi so you can do that with facetime also, etc.