Unsupported media type in nokia6120classic?

Hi, I have a problem with VOIP on my mobile, nokia 6120c. I have download (Mobile VOIP) from http://www.mobilevoip.com/symbian/MobileVOIP.sis. I have registered and I can see my money in the program, but when I call I get this message (unsupported media type). Does any one know the solution? Thanks