The standard policy is that minutes do not carry over from one month to the next. InmateAid's phone service runs on a month to month basis and each billing cycle resets regardless of how many minutes were used in the previous period. Unused minutes from one month do not accumulate into the next.
That said, if there was a legitimate reason the service went unused, whether a transfer, a stretch in the hole with no phone access, a medical situation, or something else that genuinely prevented calls from happening, InmateAid has made exceptions in the past and extended a complimentary month to account for it. The key is reaching out and explaining the situation. A polite email to aid@inmateaid.com with a straightforward explanation of why the month was lost goes a long way. The team is not looking to take money for a service that could not be used through no fault of your own.
The no carry over policy is worth knowing upfront so you can plan around it. If you know your inmate is going into the hole, facing a transfer, or otherwise going to be without phone access for an extended period, it may make sense to pause the service rather than paying for a month that will go unused. Reach out to InmateAid before the billing date and they can advise on the best approach for your specific situation.