[cpeterso]

It's built into iOS: https://support.apple.com/en-us/HT204023

  A Personal Hotspot lets you share the cellular data connection of your iPhone or iPad (Wi-Fi + Cellular) when you don't have access to a Wi-Fi network.

[nazgulsenpai]

That still requires cellular data/internet and not just voice.

[cpeterso]

Oops. I misunderstood “the phone system on a smartphone” to mean the smartphone’s cellular network, not using the smartphone as a modem connected to a landline.