The Call screen is a central place to make an outgoing call using your Private (Sim) Number or WiFi number.
Both these types work through a wi-fi or mobile Internet connection.
WiFi - an outgoing audio/video call to another TextApp user (whose number starts with '+T 123') without any extra cost. Sim - an outgoing phone call to a real phone number (using an ISD Code) by using your Credit balance.
Call > WiFi Call
This allows you to make an outbound call to another TextApp number (that starts with '+T 123'). This is handy when you want to do a video or audio call with any TextApp user without paying anything per minute, and without worrying about Credit balance.
Call > Sim Call
This allows you to make a phone call to any real phone number (landline or mobile) using ISD code. This is handy when you want to make a phone call without using any Sim card. When you make a phone call (or send a text message) using 'Sim Call' option, please be aware that your Credit balance will be used.
In case you are looking for options to recharge your Credits, here is the article.