I actually believe its not the case this time around. If they wanted to block gaijins they would do it with IP blocks. Also they broadcast the Cinderella anime for free to everyone in the west, including like 4 languages of subtitles (might not be the best quality subs but still its an effort).
The reason for the phone verification is most likely the fact that the trading is quite a big thing (idols worth hundreds of dollars) so there must be a way to limit people from starting multiple accounts and breaking the system by getting daily rewards and promo rewards on all of them.
In something like pso2 JP which I played, there you had to actually pay money to be able to sell stuff (or get very rare 3 day shop ticket). I did put some money into there, later when I was quite good at the trading I was just stockpiling items for sale (buying was free and once I had capital to buy I didn't need to sell stuff immediately) and then I sold everything like every 2 months in those 3 days of ticket, but still it was very limiting when I didn't feel like paying monthly.
So phone verification is quite a logical step, from my friend who did the verification for me I've heard that the time where you can just get random disposable numbers is pretty much gone and if you want a SIM card you have to sign up for some sort of plan and pay monthly, so everyone only has 1 number (and hence usually only 1 chance for cinderella account).
As to why they don't allow you to use overseas number for the verification - well this is not the case everywhere, in my country I can just go and pick up 10 SIMs for like 50 cents each and I can receive sms on all of them. That and it might cost more money to have the sms gate connected to non JP numbers.
So I think the cinderella guys are not really trying to block gaijins, more like they want to make sure the trading is clean and fair.
What they are really bad at though is the copyright crap, every single thing on youtube or niconico that has like 5 seconds of a song from cinderella gets instantly deleted, you can't even find most of the songs anywhere online to listen to and have to download the full album torrent for it :/
And what is even more idiotic is that they even block their own advertisement xD The Onegai Cinderella PV (PV = promotional video) has been deleted from youtube several times as well, I mean what brilliant mind came up with that idea, deleting promo vids and not even putting it up themselves xD