Welcome to Android Central! Who's your carrier? Have you asked them about the issue already?
You said, "When I try to on my data network," it shows that message. What are you trying to do? Make a call using the Phone app, or some other 3rd party calling app?
Open your Phone app, tap Menu>Settings>Call Settings>Calling Accounts, and make sure Use SIP Calling is set to "Only for SIP calls."