Most people don't know you can just dial ##004# to remove all conditional call forwards that normally default to voicemail. Or if the network doesn't unset it, you can set it to a deadend like * 004 *0000000000# - or some other fun automated line...

For Verizon at least, it is an option, but stupidly it is a paid option. I had checked 2 years ago and they wanted ~$3/month for the privilege of not providing voicemail.

Turns out it was easier to borrow a friend's phone and leave myself a bunch of voicemail to fill up the storage.