Making a call on the Phone dialer screen
How do I set the default SMS app?
If you installed more than one SMS app on your phone, you can set which app will send and receive SMS.
In Settings, under Wireless & Networks, tap
To set HTC's app as the default, tap
Messages app can send and receive both SMS and MMS and allows Group Messaging (if the device and your mobile operator support it).
and select an app.