You can download all your invoices from the Exotel dashboard. Invoices are available only for paid accounts.
Login to your Exotel account.
Click on the ₹ button at the top-right corner of the dashboard. This is where you can find all your payment and billing-related information
You will be taken to the payments page. In the left panel, you will find a link that says "Bills". Click on it
Here, you will find all the invoices listed according to the billing period. Select on the one you want to download and click on the .pdf file from the column on the right extreme.
In case you have further questions about who receives the monthly bills, how you can edit the account admin who receives the bills etc, please read this article for further clarity.