Where to find the callsigns of the en route controllers?
You can look in each country's AIP, specifically section ENR 2.1. Below is for the Netherlands:



Note the name/call-sign 'Maastricht Radar' for at or above FL 245. Maastricht UAC is...


  ... an international non-profit air navigation service provider, operated by EUROCONTROL on behalf of four States – Belgium, Germany, Luxembourg and the Netherlands.


Also from the AIP's you can find the division of the FIR's, example: