"Sorry we are unable to complete your call at this time. Please try again later."

[SEI1]

Receiving this message when trying to call certain numbers. However, when these numbers are dialed from our landline phones the calls go through without issue. I am unable to find any correlation between the phone numbers that are having this issue. Could it be a specific wireless carrier or something?

[Eliot]

are all your users having this issue with the same numbers they are trying to call?

you may want to review

Outbound calls blocked or tagged as spam – Zoom Support

[SEI1]

Thank you, it looks like one of our main outbound numbers is being flagged as spam by one of the carriers.

[Eliot]

one of my clients used free caller register to help avoid outbound calls being blocked.  this is also mentioned in the zoom support article.

Free Caller Registry | Home

federal communications commission (fcc) says "The FCC has made combatting unlawful robocalls and malicious caller ID spoofing a top consumer protection priority." 

The FCC's Push to Combat Robocalls & Spoofing | Federal Communications Commission

fcc has not been too effective.  some of the carriers use analytic services to prevent robocalls and spam.  these services sometime block legitimate calls.