https://service.ringcentral.com/faxapi.asp returns 503

Where did you get that link from? I cannot find the link from service web page.

This has been like that in our software for 10 years

As a proof, other people used to use it

https://stackoverflow.com/questions/24462555/ringcentral-php-faxout-api-example

https://community.ringcentral.com/questions/281/fax-out-503-temporarily-unavailable.html

Let me move this to the Customer community.

@Phong Vu Not sure if this is a Customer Community Thread - especially as it is specifically about APIs.
No recommendations on the Stack Overflow thread?