This is new to me. I’m sure it isn’t new to many, but I felt it would be worth a quick post. Last night I was configuring an old iPhone for a friend. In order to activate the old phone and unlock it, I had to pop in a SIM for a moment. So inserted SIM, let it activate/unlock and then removed the SIM and got to work syncing music etc. I noticed that the Voicemail icon had an alert. Low and behold there were voicemails from the other SIM, accessible even after the SIM was removed.
I’m sure my use case was unusual, but keep it in mind if you are mucking about with iPhones a lot.