Does this only happen with voicemail?

Yes, other calls were working fine

Meanwhile, the voicemail started working again for both phones about two hours later

I think it was a Koodo system issue.

Same thing happening to us on 4 lines. All phone calls seem normal, accessing voicemail hangs up. If a call is sent to voicemail, the caller gets a message saying the subscriber is unavailable.

Mine is working at this moment.  Leaving and retrieving messages.

So, when this stuff happens how does one report it to koodo itself??

There is no reporting portal for technical issues. You can report it here and we can flag a rep or a rep can see it and look into it. It resolved after a couple hours so it's good now but for example there was an issue that got flagged yesterday with people unable to pick a phone and select or ignore device protection and move past that. It has been resolved today. It started by people mentioning it here. So don't hesitate to bring up any issues here for the future !