You're still missing the point, Rose. We don't know of any other phone number that MagicJack blocks other than freeconference.com. You are wanting MagickJack to put on their television ads, web site, pre-sales chat line, and everywhere else they promote themselves that they don't allow calls to this scam conference calling service? That is ridiculous.
We also don't know if any mainstream carriers like AT&T are blocking numbers, I bet they are (for one reason or another).
Not only does it needlessly take away from the practical truth that they allow unlimited calls to any US/Canadian number, but it essentially advertises this bogus "free" conference service. Even if there were 100 such numbers that they block, for various reasons, it still would be 99.99998% of the numbers available in those two countries (assuming at least 500 million numbers).
As for your less-than-direct online chat with MJ, most people think of "conference calling" as a 3-way call they initiate on their own phone or a legitimate 3rd-party service. In which case, their answer to you was just fine. I'm really happy to see that a direct question about a specific service is answered correctly by the online chat people.
I don't use MagicJack because I don't like the USB-connection scheme, installing software, keeping my computer on all the time, and (newly discovered) the idea they might put advertising on my screen. However, it seems to be a great option for people who want to use conventional phone equipment and pay even less than Skype for their calls. I simply can't fault them for blocking access to numbers like freeconference.com which have usury connection fees, even if at the same time they are advertising "call anyone in the US or Canada."
Why aren't you more steamed at freeconference.com for not disclosing their business model and the fact that some networks "aren't compatible" with them?