Welcome to the RingCentral Community

What you are looking for is under the User - Outbound Calls/Faxes - Caller ID - and here's the fun part. RC calls it 'Alternate Caller ID'

This is the area that you WANT to make your users have an entry - as using the web/app - this is the number that comes up when you hit the number pad to dial.

Users can also select 'Blocked' if you allow them to in the Roles.

What we've done is made a template to override that field and put in the main company number, and not allow staff to change it. This way, the main CID goes out on the line.

That make sense?

We're in the same boat. Hopefully we can find a solution for this.

I think all you have to work with is
Billing > Calling Rates > Domestic rates
&
Billing > Calling Rates > Special rates

If that's not a workable solution for you. You can try your luck with reaching out to support and asking them to blacklist numbers for your account. In other words do the opposite of the Whitelisting process.
tps://support.ringcentral.com/article-v2/10362-International-Calling-Agreement-for-Whitelisting-International-Phone-Numbers.html?brand=RC_US