Elastic Calling is TextNow’s unique ability to have our calls choose the best route for your call to improve quality. TextNow tests the quality between wifi, 3G/4G and voice, and switches on-the-fly to the path that gives you the best calling experience.
You can find those settings in your TextNow Settings menu, under Calling > Elastic Calling:
Data Fallback: Allows TextNow to send and receive calls through your carrier's 3G/4G network, without Wi-Fi
Voice Fallback: Allows TextNow to send and receive calls through your carrier's voice network.
For more information, see our FAQ here.