When I tap on a phone number, why doesn't my HTC automatically call it?
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 CallingChange this setting to "Just for Internet Calls" Additionally, Change WI-Fi Calling to Disabled