we currently have a switchboard with approx 12 extensions, connected to 2xISDN2 (4 voice channels).
If we were to replace this with Asterisk, what hardware would we require & what are the main benefits of it, over our current switchboard (I think it’s an old Mitel).
This is a wide-open question, you are best off having a look at the link to the O’Reilly book in my signature below. Good luck!
Also, Digium makes hardware that could replace your ISDN setup with the Mitel coupled with Asterisk:
The question is not what asterisk can do for you, but what can you do for asterisk.