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

I just tried calling them. SNCF don’t take Eurostar bookings any more.

OR you can use the option via b-rail (google €* booking-follow page till bottom)-its 4€ extra, but you dont have high calling cost and need NO passcover nr. Till BRU you have far more choice-then its 30, to AMS just 2 trains-cost 34 or 35, BUT you can change all the time in BRU to hourly BeNeLux train onward=free on pass (but takes longer).

SNCF call centre is apparently (or maybe just 1or 2 agents whi get tired of it) not willing anymore to serve all these cheap0 passholders.

I have tried b-rail too but you need the “Pass cover number” I have a digital pass not a paper pass….

Thanks for letting us know. Did they say why?

They just told me to call Eurostar. I’m going around in circles...

They're trying to fob people off. I have the impression it's only some of the SNCF staff since many others booked there with success. Either try calling them again or try Belgian railways:

https://www.b-europe.com/EN/Contact/Contact-Centre

You can also call Eurostar but they have a quite steep booking fee.

Call again and hopefully a more service minded person will answer. 

This is the information om the SNCF website:

Press #85 for English.

Outside France

Want to contact Eurostar or Thalys, or call us from outside France?

We’ve got a special phone number just for you. If booking on line or with one of our authorized travel agents isn’t convenient, you can use this international line to do whatever you need from wherever you are.  

Dial 00 33 1 84 94 3635 and follow the instructions. You’ll pay the cost of an international call, plus any applicable operator charges.

Thank you :)