Hi Spiros,
Regarding your first question, the connection you have directly from your wall is PSTN ( analog) telephony and ISDN is digital, so yes, you need to have the box that is provided from your Telecom provider. Using an ISDN telephone as PSTN or IP telephone is only possible through a complete phone system (from Avaya, Ericsson or similar) which will make it very expensive and very much an overkill for a normal home...
I dont know about Greece, but in northern Europe, ISDN is on decline in favor for IP/analog telephony. In Sweden for instance the ISDN services for private subscribers is not possible any longer. ( I got a BEOCOM 3 ISDN if someone wants it..)
Hope this clarifies things for you...
Regarding the second question, I am not sure, perhaps someone with more insight in that product can answer...
/Peter