Parakeet is a VoIP application, which means that calls on the Parakeet mobile application will consume some of your cellular bandwidth. The good news is, they don't consume much.
The Parakeet mobile applications intelligently switch re-negotiate a more data-friendly connection when they detect that they're using cellular data. In this case, Parakeet will prefer to reduce the audio quality in favor of saving bandwidth (you can disable this in the app settings if you'd prefer).
When Parakeet detects that it is connected to a cellular network, it consumes approximately 8.8 KB/s of data. On a typical 2GB data plan, this equates to approximately 64 hours of talk time. However, it's rare that you'll spend all of your time on cellular. Generally, when your phone will connect to Wi-Fi networks where available, and Parakeet will automatically switch the call over to Wi-Fi to save data.