Please see the screenshots beneath. When my grandma tapped 'Mobile' beneath in screenshot #1, her HTC Desire 825 failed to, but ought've, instantly connected. Rather, a "No Internet calling account" message appeared.
But when she input that 647 number manually, her phone connects perfectly as in Screenshot #3.
Do you have an application called "Voice Changer" installed? I've heard that it can cause this. Likewise, if you have google voice installed, do you have it set to "Never Dial with Google Voice"?
If those are not the reason, do you have any other "Telephone Linking" applications or "Sip Dialers" etc, remove them and check whether your issue stops.
If none of these options above work go to:
Settings > Call > Use Internet Calling
Change this setting to "Just for Internet Calls" Additionally, Change WI-Fi Calling to Disabled
Q & A