Whenever you have a conversation over text and you don't see an amount of money in the chat discussion, it means your exchange or conversation is not getting charged. The same when you finish a voice call and you don't see an amount being charged at the end. Finally with questions, if you don't see an amount when you answer the question, it means you are not getting paid for it.
The reason for that is that the user asking you a question, chatting with you over text or calling you is on your list of the users that can contact you for free.
If you didn't add that user previously to your free list, it means that this user was added by default as they previously had your phone number details in their phone book. By default on Instantgo, any users that previously add your phone number can call you for free on Instantgo as we assumed that could have done it for free through a regular phone call.
In order to change this, you simply need to access your free list and remove them from it. Go to Settings, click on the Contacts list and then select Free list. In a chat conversation, you can click on the Settings wheel on the top right and decide to change if this person can contact you for free directly from that window.