How do you have those 3000 numbers saved? Is it an excel spreadsheet?

i know RC has some destinations which are blocked by default for outbound calling and you have to whitelist them by submitting a form in order to reach those numbers. I am referring to this:
https://support.ringcentral.com/article/5150.html

Maybe you can try contacting support and have them "blacklist" the numbers you want for "inbound"... in other words block them for you. Not sure if this will work, but it's the most convenient thing...

If you decide to give it a shot... I suggest you follow the instructions in the support article above and just create a case using this pdf (step 1 of that support article)
https://assets.ringcentral.com/us/guide/Whitelist-Customer-Agreement.pdf

I know it's the wrong document but you got space to write and you can briefly describe what you want. The good thing about using this "wrong document" is that your case will land in the correct department which does the "whitelisting" and the "blacklisting".

Like I said I don't know if RC will help you with this one, but worth a shot imo.