You first need to go through documentation rather than asking random questions.

Ringcentral api expose fax API which you can integrate with the programing language of you choice.

You need to install different SDK of your choice.

For example, if you want to do it with php then there is RingCentral PHP SDK and copy the demo code from the link below.

https://github.com/ringcentral/ringcentral-php

https://github.com/ringcentral/ringcentral-php/blob/master/demo/fax.php

You need to create an application at https://developers.ringcentral.com and add the Fax and any other required permissions to your app.

A number of SDKs also have built-in fax request helpers including:

• PHP on Packagist: https://packagist.org/packages/ringcentral/ringcentral-php
• PHP on GitHub: https://github.com/ringcentral/ringcentral-php
• JavaScript on GitHub: https://github.com/ringcentral/ringcentral-js#fax

There is Ruby SDK documentation:

• Ruby fax docs: http://ringcentral-sdk-ruby.readthedocs.org/en/latest/usage/messages/Fax-Messages/

You can also go through these to get better idea:

https://forums.developers.ringcentral.com/questions/144/fax-faq.html