Hello,
If you are using Zoiper Classic Free, according to our terms and conditions(https://www.zoiper.com/en/zoiper-general-terms):
“Free License” shall mean a world-wide non-exclusive, non-transferable, non-assignable, in whole or in part, and perpetual right to install, activate and use one copy of the Product only for demonstration, evaluation and entirely non-commercial purposes on a single device, including to load the Product, to display it on screen, to perform, transmit and store it in the memory of a device as much as this is necessary to achieve the intended purposes of use of the Product, but without the right to translate it, adapt it or make other changes to it, including to debug it, for any reason"