It may help explain how missed call alert & your call log work - what you're describing appears to me, perfectly normal behaviour from a voicemail perspective.
1: Call rings on your phone and you miss it, caller leaves a voicemail. You receive text notification of the "new message received" (assuming they're enabled), the voicemail spool on your phone's screen screen is activated and the "missed call" shows in your phone's call log.
2: Call rings on your phone and you miss it, no voicemail is left. Your phone's call log is the notification.
3: Call doesn't ring on your phone and a voicemail is left. Same as scenario 1, minus the "missed call" listed in your phone's call log.
4: Call doesn't ring on your phone and no voicemail is left. A "missed call" text notification is sent, assuming they are enabled.
Perhaps you could post a screenshot of the notification you received, to enable accurate diagnosis of which of these scenarios, applies. I suspect option 4 does.
As to why your phone didn't ring, that's another separate issue. Being on fibre broadband is completely immaterial - what matters is whether your phone had a mobile network signal at the time, or an active WiFi-calling connection. If you test calling your phone from another phone now, does your phone ring as expected? Have you had problems with any other calls not ringing when at home?
