We deduct the rental charges immediately after the ExoPhone or the coworkers are added. The charges will be deducted only if you add a Coworker or an ExoPhone added beyond your account's free limit. The recurring monthly rental for these additional users and ExoPhones will be deducted on the 1st of every month. Let's say for example(this applies to both adding coworkers and ExoPhones), your account has a limit of 3 ExoPhones which will be free and you would like to add another one on the 15th of a month. You will be charged the rental on the 15th for that month and the monthly rental for the next month will be deducted on the 1st of the coming month. If you choose to delete the ExoPhone before the last day of the same month, you will not be charged the rental for the next month.
The Rental Charges tab under the Buy Credits page will give you the summary of the monthly rentals applicable to your account as shown below: