For 3g2 to webm conversion you can use for example the Free Studio, or Super converters for Microsoft Windows. On other platforms there is available a cross platform FFmpeg converter. To convert 3GPP2 files to WebM format open command line and type:
ffmpeg -i /path/to/your/file.3g2 -vcodec copy -acodec copy /path/to/your/newfile.webm