What you need is a Voice Over IP client app (AKA 'softphone') for Debian, such as one of these listed at https://www.linuxlinks.com/voip/ and https://www.ubuntupit.com/top-20-best-linux-voip-and-video-chat-software/ (there are many more).
You may also need an account with a telephone provider. Depending on where you are, Google Voice may be free (US & Canada, IIRC) if you have a Gmail or other Google account, and works well if you launch it from within Chromium or Chrome.
Skype allows outbound calls for free the last time I looked, but getting a PSTN ('Public Switched Telephone Network') number was not free, so you would be limited to other Skype users unless you paid.
The last time I had a paid account for PSTN access not with Google or Skype, I paid by the minute plus per call connection plus a flat monthly fee, but it wwas very reasonable.
Your WWAN provider also may block softphone calls; it's up to their Terms and Conditions.