Their launch tagline was "Open source alternative to Twilio".
I just took a look at their repository(https://github.com/plivo/plivoframework) and it looks like it hasn't been updated since 2015 and the documentation has been down for long time(https://groups.google.com/g/plivo-users/c/ZuImvGKhnno/m/Uj-109PQCQAJ). I couldn't find any announcement that they have become closed source. I think I might have missed a conversation/announcement regarding this.
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Ask HN: Did Plivo silently abandon open sourcing their software? | Heykuki News