There are tons of companies in this space; have you considered actually doing the "pay for the service" plan and evaluate these options? What did they not do that you wanted?
I search for a replacement every year. I have never found a near competitor. Vonage used to have a desktop app that was at least in the ballpark, but they dropped support years ago.
Could you please list some? I've looked, and I did not find tons, and I was actually less convinced of the quality of what I found than of Voice! It's gotten to where I considered rolling something with Twilio et al.
