If you can't place a call, the developers recommends you do a clean install. Remove the application from your phone, then re-install it using the Play Store application. There is a dedicated page created by the developer to come in need to those who have issues running the application or calling.
Support page: http://snrblabs.com/snrb/Apps/GrooveIP/FAQ.aspx
You can try another thing in your phone related to network settings. Try changing your network source from WiFi to Data and backwards. It's possible that some networks don't allow Internet calls or they have all the ports filtered which makes it impossible to call.
