EZ Route only supports a single file format for file uploads, which they refer to as "VOX ULAW". There's little information out there when referred to in that way, and EZ Route documentation is sparse. More correctly, the file format is Dialogic ADPCM. If you're really interested in the format, see this site.
But I'm not. I just want to convert WAV files. For that, Sox is the man. Here is the magic command line for conversion:
sox input.wav -t ul -r 8000 output.vox