That depends on how you use it. Normally only your phone's internet connection (Wi-Fi, LTE, 4G, etc.) will be used. Some older versions of the app support "Bridge Calling". If you use this option, then your cell provider will bill you according to the plan you have with them. This will also happen if you forward calls to another number.
When sending a text message, we will not use your text message plan from your cell provider. However, we will use your data connection, which may lead to normal usage charges.