Cell phone carriers use analytics companies to rate numbers. Numbers are given a rating from 1-10 for example:

• If the main number, has been given a rating of 4 with the cell carrier (AT&T, Verizon, T-Mobile etc...), this would mean the number will show up as spam, but still allow the call to go through.

• If the number has been given a rating of 10, the number would be tagged as spam and the call not be allowed to go through at all.
  

This blocking or labeling as SPAM is done on the receiving provider side. As a result, they will have to whitelist the number to clear the issue.

These are the sites where you should submit the request for the number to not be treated as SPAM.

T-Mobile
https://callreporting.t-mobile.com/

Verizon
https://voicespamfeedback.com/vsf/

AT&T
https://hiyahelp.zendesk.com/hc/en-us/requests/new?ticket_form_id=824667

You can also try going to this website and registering the phone number, so it is no longer tagged as spam.
Please note, that this is not a Sangoma-related website.
https://www.freecallerregistry.com/fcr/