A footnote: A friend in the US with relatives who reside in France has determined that the "country" blank is not required, and that regardless of how it is filled in (or not), you can enter a VALID French telephone number in the phone number blank and the site will accept it -- including for an English-language guided tour. My friend used the number of a relative in France and it worked.
Testing ,y lack of success, my friend also tried various versions of both of our US phone numbers and none worked. But without knowing whether a ticket and reserved time would then be texted to the relative's phone number or would be emailed to my listed street address, we were not willing to proceed with an order. (My friend's relative has no interest in an English language tour of the Opéra Garnier.)
And, for Susan, no, we did not try a making up a bogus French phone number, which might have produced a wasted "ticket-into-the-ether" result if it texted the response to the phone number.
Finally, none of my emails to the site's "contact" email address have been answered.
Bienvenue à Paris, si vous êtes français.