It depends on which type of service you purchased, and the two main options work differently when it comes to refunds.
If you purchased phone minutes or deposited money into a calling account and those funds went unused, most carriers will refund some or all of the remaining balance. The specific refund policy varies by carrier, so contacting them directly with your account information is the fastest way to find out what you are entitled to and how to request it.
If you purchased a phone number through InmateAid's discount service, the situation is different. A phone number is a month-to-month service where you are paying for the line itself, not for individual minutes used. The line being available and active is the product, regardless of whether calls were made on it. That type of service typically does not come with a refund for unused periods because the commodity is the number, not the usage.
If you are unsure which type of service you purchased, log into your InmateAid account and check your order history. That will clarify what you bought and point you toward the right contact for any refund questions. InmateAid's customer support can also walk you through your specific situation if the account details are not clear.
Going forward, if your inmate's situation changes and the service is no longer needed, canceling a month-to-month number before the next billing cycle is the best way to avoid being charged for another period you will not use.