The 300 minutes are not free. They are an allotment, meaning the Bureau of Prisons sets a monthly cap on how many minutes a federal inmate can use, but every call within that allotment still costs money that comes out of the inmate's account.
Here is how the financial side works. Federal inmates manage their funds through TRULINCS, which is the BOP's internal banking system. Money deposited by family members on the outside flows into the TRULINCS account, and from there the inmate directs funds toward phone calls, CorrLinks email, commissary purchases, and other approved expenses. There is no free phone time built in. Every minute has a per-minute cost that gets deducted from the account balance.
The rate that applies depends entirely on the number being dialed. Long-distance numbers cost $0.21 per minute. Local numbers, meaning numbers with an area code geographically close to the facility, cost $0.06 per minute. On 300 minutes a month, that difference works out to $63 at the long-distance rate versus $18 at the local rate. The savings of $45 per month is real and consistent every single month.
InmateAid provides a local number for $8.95 per month (Now $5.00/mo) regardless of where you actually live. Your inmate dials the InmateAid number, which qualifies for the $0.06 local rate, and the call routes through to you. Net monthly savings after the line fee: $36.05 (Now $40.00). On an annual basis, that is over $430 staying in the commissary account instead of going to the phone carrier.
If you do not already have an InmateAid local number set up for a federal facility, it is the single highest-return change you can make right now.