You will need two things.

1. Google Voice Number (yes I know, Google)

2. Wi-Fi calling

If your phone plan expires for whatever reason and you think you can't reach your voicemail again to see if anything was missed. You can call the voicemail itself by jotting down the number of which you call your voicemail by and input your password to that voicemail through Google Voice. I just discovered this yesterday when I couldn't use my phone because my minutes were up.