GetOnSIP by Junction Networks is another free service. Initially launched as a SIP service it’s evolved into a hybrid of SIP and WebRTC that should be very useful to podcasters. There exists an entire universe of SIP clients, both hardware and software, making such services eminently accessible.
Using SIP-based infrastructure for a podcast makes a lot of sense to me. It allows you to use SIP URI’s as addresses to bring guests into your call. They look a lot like email addresses, which makes them easy to understand for non-technical folks. The fact that you can pass someone a link that will work in a plain vanilla SIP client, without requiring registration to a SIP server, makes things fairly easy.
There are a diversity of free and cheap SIP clients for any platform you care to mention. Some SIP clients, most commonly soft phones like Jitsi, Blink or Bria, include call recording, a useful feature for podcasters.
The great thing about SIP services is that they allow you to leverage SIP hardware if desired. I still like my Polycom VVX desk phones for their great sound, convenient on-phone call recording and ability to conference between SIP services and plain old PSTN callers.
Further, by staying to IP-based services you can leverage the family of audio codecs that give your HDVoice. Whether baseline G.722, G.722.1, SPEEX, CELT, G.719 or Opus, you’ll have audio quality that rivals traditional broadcast radio.