Our accounting cycle is the 1st of a month to 30th/31st of that month. So, usually by the 5th of every month, all admins of the account will receive an email that mentions what balance was present on the previous 30th/31st, what are the different events for which the balance was added or reduced in this billing period, and what is the current balance is. This email is computer generated and hence can be printed and used as a bill.
Furthermore, every time you make an online payment, you will instantly receive an email with an acknowledgment that can be used as a payment receipt.
You can find all your bills in your Exotel dashboard.
Click on the Rupee symbol at the top right corner of the dashboard. Then click on the "Bills" link in the left panel to find a list of all your previous bills.