Looking at the UGS Pricing page …
Vivox itself seems to be free up to 5000 PCU (peak concurrent users) but the safe text feature is listed as free up to 5000 MAU (monthly active users).
If this is correct a game with 20k MAU, unlikely to get anywhere close to that PCU limit, would still incur charges of $0.05 * 15k = $750/month.
Please confirm this is how billing will work for this feature. 5c per player for a free to play game is a significant chunk of potential/expected revenue!