Hi, faxes are sent individually to recipients and may take 1 minute per page. You may check these KB Articles to know more about Fax Transmission.

https://support.ringcentral.com/article/Fax-Troubleshooting-Checklist.html

https://support.ringcentral.com/article/Factors-Affecting-Fax-Transmission.html

Basically, every user extension under your account (company) will share the same fax number (which usually is your main company number). Thus, every fax sending out thru the same number will be on the same queue.

For your use case, I suggest that you should add more phone numbers (let's say 3 more plus the current main company number to match your 4 lines). Then you should create 4 dedicated extensions (let's name them "Fax station 1", "Fax station 2" etc.. Each "Fax station" extension will be assigned with a different fax number.

Set Fax settings for each extension

Select Fax number for each extension

Now it's up to you to implement your fax app to select a "Fax Station" extension to send out a fax. You can query fax messages status of each "Fax Station" to see how many faxes are on the queue of that fax station and decide which one you want to use for sending a new fax.

Thank you Phong Vu. Your information and instruction are very helpful. We also need to know if we can customize and use our own fax cover page instead of the built-in cover page from Ring Central. I read an article from RC community about custom cover page from Ring Central App. Not sure we can create and use custom cover page with Fax API. Your help will be greatly appreciated.