How are the usage minutes for a seat calculated?

Usage for seats is pooled at the parent account level.

For example, if a parent account "A" has 10 telephone numbers (TNs) with 500 seat minutes each, the total number of minutes for this account would be 10*500 minutes = 5000 minutes. Although each TN on account "A" has a total of 500 minutes, the overage (or excess) minute used charge is only applied when the total of 5000 minutes is exceeded at the account level.