[inquisitor]

Never heard about these guys. According to their FAQ section the SIM card is based on a Mobilkom Liechtenstein (like United Mobile once upon a time) but has multiple IMSIs. One of them comes from eplus Germany, which explains the low price for the 5GB data pack for Germany mentioned in the article you linked but not offered on their website. Obviously someone has seized United Mobile's last plans, which involved a multi-IMSI SIM and they even already had an agreement with eplus back then. I also remember that UM sought a code monkey specialized in VoIP.
Also "booking a foreign number" seems not to work yet. Anyway I wonder how they want to offer American numbers. With the American bill-and-keep arrangement they won't get a penny for terminating incoming calls, so they will have to charge their customers something for inbound calls at least for US numbers. Even for those countries with the calling-party-pays scheme they will need mobile or premium numbers in order to provide free incoming calls.
Despite the SIM being offered by a German company which according to their T&Cs is also the contractual partner for the actual mobile service and as such subject to EU regulations they violate the price cap at least for Bulgaria. Those € 0.05/min charged for incoming calls in all the other EU countries I checked, are pretty cheap, like rates for many other countries are.