Well I can tell you that any SIP program can do this, one that comes to mind is Linphone. But you do need a provider. Almost all providers deal with UDP. You might be able to find one that transmits over tcp but that’ll be a very small list. TCP isn’t reliable for VoIP, for media anyway. At best you’ll find one that encrypts the signaling over TLS, which is TCP. But audio, even if encrypted too, is done over UDP.... Source: over 1 year ago

Does Linphone (https://linphone.org/) not meet that need of cross platform (website claims versions for all major OS'es) and open source (https://gitlab.linphone.org/BC/public/linphone-desktop)? - Source: Hacker News / over 1 year ago

Similarly, you can use any SIP (VoIP) provider (e.g. VOIPo Cloud), and configure the SIP account details in your native phone app. This way you will also be able to use a SIP softphone on your computer (e.g. LinPhone on Linux or Telephone on Mac), or even a real SIP phone in your home. Depending on your SIP plan you may or may not be able to make international calls this way. Source: over 2 years ago

I heard on RFD Fonus is using linphone.org to code their service. Source: almost 3 years ago

Excellent! Glad we could get you sorted! Hosting can be scary and it's okay to be afraid to touch things or not understand certain settings or terms. Especially if you're new. The key is to read the documentation. For your forum needs, this can be found at https://mybb.com and https://phpbb.com. Source: 7 months ago

You may find a current, object-oriented version of phpBB to be just the ticket. It will teach you how to structure the database, authenticate users, manage sessions and selectively display content according to user level, group membership, and other policies. Source: about 3 years ago