This is super strange but not unheard of. It's possible that the call went directly to voicemail instead of ringing then going to voicemail. If that's the case, you wouldn't have a missed call notification but instead only a voicemail notification. Did this happen at home? Were you connected to WiFi? Lastly, has this happened again within the past two days?
Has this call log issue been resolved? Please give us a reply and let us know.
Were the voicemails from callers you know, or advertising?
There's this insidious practice called "no-ring voicemail broadcast" that some
marketing companies are doing. Ex: opencallcentre.org ... if you like seeing
just how sleazy such people can get. It requires a certain type of arrangement
with the target telco(s), usually a server connected to the internet that the
senders can dump audio messages and recipient numbers straight into,
that eventually gets to the voicemail server. At least that's how I last
understood how it worked; I haven't looked into this for a while.
Hopefully T-Mo prohibits such practices and does NOT accomodate that
nonsense, and what you saw was just a network hiccup that had you
offline when people called...