In most cases, when you make a call from the Onoff app, your SIM card takes over to make the call, so there is no loss of quality.
When you receive a call, by default, the application prompts you to take that call using the Internet: this is called a VoIP ("Voice over IP") call. Depending on the state and quality of your 3G / 4G network or your Wi-Fi connection, it is possible to observe differences in terms of quality of call reception.
To avoid any quality problem, know that you can at any time go to the setting section of your application, to also switch incoming calls to your SIM card by disabling VoIP calls. Simply deactivate the option "Reply via Wifi / Data network"