I found that odd too. I live in Denmark and i pay a fixed amount and can call as much as i want.
Those fixed price for unlimited calling plans that are great for human to human communication would suck for low data sensors and devices.
In the US these are handled by special plans that have zero or close to zero fixed monthly cost, but have a high per byte or per minute rate so that they are quite cheap for their intended use (e.g., power meter daily usage report) but are too expensive to use for high data applications.
If there are similar things in Europe, maybe that number was one of those?
They are great if you have sensors travelling to different countries or your volume isn't large enough to negotiate with a local telco.
If you are deploying many devices in one country it will be far cheaper to talk to a local telco about getting a custoomized data package for your SIM cards.
https://eseye.com is another company supplying these roaming SIM cards.