https://wiki.sonic.net/wiki/Fusion_Voice_Mail notes that it is possible to set the number of rings before going to voice mail. But the web page manager does not allow for number of rings to be entered, but number of seconds. Rings would be easier, I think.
Hmmm. I enabled voice mail on my line, and I'm asked for the number of rings.
I have traditional copper Fusion. Perhaps you have FTTN with VOIP? If so, the settings may be different.