You can create separate Conversations in Chat-Flow to store the Accept/Reject value. When the template message is sent to the user, the next response from the user should trigger the respective conversation which will be stored in the defined property.

For example :

You can create a conversation "Accept" with the starting rule as shown in the image below,

In the above conversation the property "Call permission Accepted" is updated so that you can differentiate the users who have accepted the order from those who have rejected.

Similarly another conversation for "Reject" can be created.