opal shale
The usage-based billing ($0.18 per 1000 characters) is for the character quota; how many characters you can use. On the Growing Business, you have 2,000,000 characters, and if you run out, you can pay to get more quota and you pay per 1,000 characters extra you use.
If you reach the voice operations limit, you will have to wait until the next month to do any voice operations again. The only thing you can do, unfortunately, is to wait until next month when it resets.
old crane