You should be able to book by phone at SNCF. No need of Pass Cover Number. 

https://www.sncf.com/en/customer-service/contact-us/telephone

 Press #85 for English, no booking fees, reservations are sent via e-mail.

If you want your Pass Cover Number you need to contact Customer Support through this form and asked for it.

https://eurail.zendesk.com/hc/en-001/requests/new

Customer Support is currently overloaded with requests so you will probably have to be patient to get help. Please let Customer Support know what date your travel will start so that they can prioritise your request correctly.

Thank you very much for the help, calling them directly worked however I slightly confused why someone should take the mobile pass if it means its harder to buy tickets. 

The biggest advantage (for me) of the mobile pass is that you don't have to specify your start date when you buy it.

After buying a mobile pass, I immediately request a pass cover number from customer service, even if my travel date is months away, so I have one in case I need it.

I do exactly the same

For completeness, you can now request a cover number directly from the website without any customer service contact: https://www.eurail.com/en/book-reservations#/generatePassCoverNumber

So helpful, thank you !