I am looking (still) for hardware for a phone center which will connect to 2 ISDN lines (as in 2 x BRI, equals 4 ‘lines’) so I suppose maybe 4 FXO interfaces and 8 FXS for connecting plain old telephones.
Please suggest some cards that I could use.
After a little searching in the net I found Digium TDM 2421B and TDM2421E.
Are these appropriate? Maybe I should use something with BRI interface and add FXS ports (how?!)?
Btw is there any online reseller in Europe which has good availability and reasonable pricing?!
Thanks in advance 
in theory you could use a TA in between Asterisk FXO interfaces and the ISDN lines, but that would be a novel approach.
there are plenty of ISDN interfaces available for use in an Asterisk server … cheapo HFC or AVM Fritz! cards, Eicon Diva Server PCI cards with DSPs, Digium b410, Astribank BRI USB boxes … features and quality vary, but they’ve all been used extensively.
to connect your analogue telephones you could use a card such as the Digium TDM2400, or the Sangoma AFT A200, or a channel bank and E1 card, or even an array of ATAs like the LinkSys PAP2.
where in europe are you ?
Greece!!