There are tons of VoIP number services that you can use through the internet. The quality will probably suck and you’ll have to figure out how to set up a SIP client, but they’re cheap and widely available.

If your carrier and phone support WiFi calling, there’s a good chance any local WiFi network will be basically free of cost. Make sure WiFi calling is enabled and reported as ready if you want to be sure it won’t cost you, but it should just work if you have it enabled.

I’ve put my phone into airplane mode, turned WiFi back on (not the mobile radio) and called my voice mail number. It connected without issue and even showed me a little WiFi icon in the call screen to indicate I was using WiFi calling.