The
Eumex 200 Manual is available online (but in German only).
"Easiest" way to do it is probably straight from the telephone - you need the bit about "Von einem Telefon konfigurieren", and hope that the PIN hasn't been changed (i.e. is still 0000).
By the sounds of it you just want to delete the programmed MSNs (if you don't know what that means, you'll have to hope I know what I'm talking about here!) - the steps should be
1. start config mode: lift handset, press the R-button, dial 8, *, 0, 0, 0, 0, (hear the positive signal), replace handset - if the PIN has been changed then I guess you'll get a negative signal (whatever that sounds like)
2. delete all MSNs: lift handset (hear internal special dialtone), dial 9, 0, 3, #, (hear the positive signal), replace handset
3. close config mode: lift handset (hear internal special dialtone), dial 8, #, (hear the positive signal), replace handset
on any incoming call, all attached handsets should then ring - if you want a more complicated setup then I recommend a crash course in German and ISDN technology!