Let us explain this by taking you back to the sign-up process. When you signed up for an Exotel account, we credited your account with Rs. 1000/- that can be used to test out our system.
During the sign-up process, we asked you for an Exophone to use in your testing.
We also asked you to add a co-worker for the purpose of testing.
Both of these have a daily rental attached to them. The daily rental is Rs.75. This rental is deducted from your initial credits as well. This is the reason you have run out of credits.