Many operators are offering travel-type SIM cards (SIMtravel, TravelSIM, WorldSIM, Lycamobile, Lebara, GoodLine and many others).
All these SIM cards most usually have one thing in common - "free" incoming calls in roaming in different countries.
While this may seem attractive, there are drawbacks of using these SIM cards. You are getting a new so-called premium phone number (Estonian, UK, Netherlands etc.), which means operator who is providing the SIM card is charging other operators for calling their number.
In usual call scenario, a person who is calling your premium number would pay for this call (that's why it's "free" for you).
In case with Roamer, the operator that is calling this SIM card is Roamer (as you keep your regular number instead of getting a new one). This applies to outgoing calls as well, as Roamer uses callback technique to connect calls. As the calling party is Roamer, the extra call charge is debited from Roamer user account - it might vary from 20 to 30 eurocents per minute.
Still, you may enjoy cheap Roamer rates even when using this type of SIM card, but only in case you are using Internet-type of calls.