My employer is now having lots of audio/video conferences, some of which I should at least listen to. Unfortunately, they are doing the conferences with Cisco Webex. Webex uses an app that's not available for Debian Linux. I've tried Webex's test site and have concluded that getting the full audio/video conference is not an option. Mostly (so far) I don't need video anyway. For audio only, Webex does let me call in to a toll telephone number and enter an event code. That works.
To avoid the phone charge, I can use a cellphone that has unlimited free minutes. But I'd rather sit at my desktop. It seems VoIP should let me connect to the toll number for free. There are many Linux VoIP programs, but I haven't tried them yet. Do such programs require everyone at both ends to be running the same program (as is done with Skype)? Can you recommend one or more VoIP programs that allow toll-free access? I'm running Stretch with a simple window manager (fvwm), not Gnome or KDE. Thanks.