In VoiceXML the goto tag is used to go to another form only. I doubt that next_page.vxml is the name of any form you created. Use this instead:
<submit next="next_page.vxml"/>
Or use the name of a form in the VoiceXML document.
To find out more about the support for VoiceXML in UCMA read this page http://msdn.microsoft.com/en-us/library/lync/hh243693%28v=office.14%29.aspx (it states that VoiceXML 2.0 is supported, so the goto tag should work when used correctly).