Your Exotel account is a prepaid account - This is similar to the prepaid SIM card mobile number connection offered by telecom operators. Your account will be active & all functionality will be available as long as your account has not expired & there is available balance in your account. That is,
Validity: These are pre-decided denominations that you can purchase which will extend your account's validity. If you purchase a pack that gives 6 months validity today (Say, 1st June 2016), your account will expire on 31st November, 2016. Lets say you paid Rs. 9999 for this 6 month validity, you get 4000 Exotel credit in your account just like how SIM card validity recharges will also give you talk time (Ofcourse, these days most prepaid SIM cards have 10 year validity by default. So, no one really does validity recharges anymore). If you do another 6 months recharge one month down the line (On say, 1st July, 2016), your expiration date now will be 30th May, 2017 because your earlier validity expiry date was 31st November, 2016 - extension will happen from the date of validity expiry. If you recharge any time before your account expires, then you will carry forward the pending balance. On the contrary, recharging an account after it expires will mean you would have lost all unused pending balance. By losing, we mean that you will see a line item for "services" costing you the pending balance in your invoice for the expired month.
Topup: Topup is useful if you are running out of credits yet there is sufficient validity. Using the above example, lets say you run a campaign on 25th of August that cost you Rs. 5500, then you only have just 500 Rs left. You would now "Top Up" your account for, lets say Rs. 5000.
Refunds: Currently, we do not refund your prepayment. Read this article to find ways to consume unused exotel credits.
Temporary suspension: There is no way to temporarily suspend your account.
Tax: Would be applicable as per the Governing law.
Exotel's pricing works like a prepaid card or a wallet. Once you have credits - You can use the credits for a variety of things: Making calls, Sending SMS, installing apps or adding users. Applicable charges for these activities are mentioned visibly.