Unfortunately, all businesses using SMS regardless of volume or use case are required to register to access SMS.
If you are sending less than 15,000 SMS/mo and less than 2,000 SMS to T-Mobile recipients the fees would be:
$4 brand fee (one-time, charged by TCR)
$15 campaign vetting fee (one-time, charged by Syniverse)
$1.50 low-volume campaign maintenance (monthly, charged by TCR)
These fees would cover all of your numbers.
If you send more than 15,000 SMS you would need a high-volume campaign that costs ($10/mo). If you send more than 2,000 SMS monthly to T-Mobile recipients, you would need to have additional vetting ($40 one-time) otherwise T-Mobile would block messages over the 2,000 "non-vetted sender" limit.
Here is a more in-depth breakdown of TCR fees. If you have any questions please let me know. https://support.ringcentral.com/article-v2/TCR_registration_fees.html?brand=RingCentral&product=MVP&language=en_US