An example of software for audio calls with the simplest possible interface (for an elderly person on a laptop)?

Question

Telephony

A person does primitive things on a laptop (watches movies, etc.) and sometimes you need to help with voice remotely.
The smartphone is inconvenient - one hand is busy, etc. I was about to set viber - but he suddenly asked me to confirm the binding to the smartphone after a while - it's not good.

that is, the requirements: the ability to make an audio call, a simple interface, free, ideally both on linux (for me) and on windows (remotely), does not require registration via sms

Answer 1

Skype.

Answer 2

for audio calls with the most simple interface

viber

The smartphone is inconvenient - one hand is busy, etc.

Headset
In general, there is a teamviewer for remote assistance - both access to the screen and voice chat if necessary.