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In that case, you can install the 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 have to create an application at https://developers.ringcentral.com and add the Fax and any other required permissions to your app. https://developer.ringcentral.com/library/getting-started.html

If you want to send fax directly from your front-end web based application, you have to use JavaScript SDK and implement 3-legged OAuth (https://ringcentral-tutorials.github.io/oauth-js-demo/?distinctId=1398916020)

+ Phong

I am using php for web based application and trying to integrate fax api into my code.

What do you mean by "integrate fax API"?

If you are not able to call the Fax API from your app, let me know what programming language do you  write your app so I can provide you with some code snippet?

+ Phong