It seems a lot of people have the same problem of trying to increase the number of rings on their Straight Talk Home Phone. I first tried going to *86 voicemail setup, but couldn't re-configure it there.
After chatting with a rep, here's the answer I got:
"Be informed that this default action or settings are determined by the carrier's network that the phone is currently working with and this could not be changed on our end. I am sorry."
So no go. If anyone finds an answer, please let us know!