Verizon Digital Voice-Personal Greeting Poor Quality?

[RavenManiac21]

I'm trying to upload a professionally recorded, crystal clear high-quality 44100 Hz, 16-bit, mono wav file as a personal greeting for my Verizon Digital Voice system. Per Verizon's instructions, I saved the file as an 8000 Hz, 8-bit, mono u-Law wav file, but the uploaded file sounds like absolute crap with a lot of hiss and noise. I don't know if this is due to the converted file settings or the Verizon voice system.

What is the highest-quality audio file I can upload to the Verizon system and what settings should I be using? Thanks!