The original thread on this topic closed due to inactivity without an answer - I found it because I was having the same problem, the glitch is apparently not fixed. I experienced a similar problem on another site I cannot recall and the fix in that case was also to use the French language version of the site that handled the US phone number using the country ID pulldown (US flag) plus the plain text 2015551212 mobile number.
For FNAC, which was being extremely difficult I went straight to the fnac.fr home page and created my account in the French version of the site without any difficulty, using the number format above. If your French is nonexistent you can let Google translate for you.
Once you are logged in (and have acknowledged the confirmation email) you can navigate to museum tickets using the main menu selection (pull-down) to navigate to Tickets\Museums and pick your tickets.
One final thing that had me stumped is that when I tried to check out I had to click on a grayed out option to bring up the delivery method selection tab - to choose pick-up at site or electronic delivery - completely counterintuitive (for me) since it could have been made a step in the process instead of a cryptic message saying that I needed to enter the information.
Hope this helps!