There is no such additional charge when you use WiFi calls & Chats - through any number that starts with +T 123.
However, to prevent the TextApp network from automated DDoS and other spam attacks and keep the service running throughout the world, we have applied a fair usage policy which can be found in the Terms of Use - also it should be visible in terms.
When you use Sim Calls & SMS services within TextApp, you will need to maintain a never expiring Credit balance. The Credits will be consumed based on destination number, that too only when you make an outgoing activity through Sim (Private) Number. Incoming is certainly free!