To help you out here, we would need to confirm few details with you.
1. Is the OS on your phone up-to-date?
2. If you have a dual-band Wifi router, did you try the Wifi calling on both Wifi 2.4 and 5 GHz bands? Depending on your
3. Have you tried restarting your phone or, as a last resort to restore the phone? For instance, you could try to disable the Wifi calling option, restart your phone then enable Wifi calling back.
4. Have you noticed any issues with the Wi-Fi connection on the phone (the connection may be resetting)? For instance, the Wifi connection's indicator or the Wifi-calling symbol disappears?
Most mobile phone have an option to reset the network settings, this could also be possible solution. You will then have to reconnect your phone to your Wifi network manuellement since all mobile, Wifi and bluetooth related settings will be reset.
For any difficulty, we certainly recommend to contact our technical support for further troubleshooting : http://www.videotron.com/residential/contact-us.