Hi & welcome!
It’s been stated by MSM that SS phone was “turned off around 8:30a.m.” & it has also been stated by MSM that SS phone “was no longer on by 8:30 a.m.”. I noticed most local news sources said that LE told them the phone was turned off, but there isn’t a direct quote from LE saying this.
I think that could easily play either way & as mentioned already, the phone would be needed to know whether it died or was turned off.
In general, an iPhone (which is what kind of phone SS had) will NOT ring at all before going to voicemail, regardless of it was turned off or if the battery died. The same with being out of service. The only way it would start to ring at first before going to voicemail is if it was still on & someone declined the call, but this was never reported & is just an example I’m giving.
IMO, finding the phone is the only way of knowing anything further about this.
Another example I’ll give is an app I personally have on my phone to block some spam calls. You can set it up where it appears that your phone isn’t on, is out of service, etc. You can send possible spam calls to hear this message or to voicemail. You can send all calls to this or just certain numbers.
My point being that it’s really impossible to know. Many factors could come into play, although some are obviously more likely than others.
ETA: Sorry so long! I was talking to my 6 year old while typing this & starting rambling!