1st of all, and most important of all, you must have the telco convert the 24 analog lines, to a single ISDN E1 PRI line.
That is not optional, is mandatory. If you don't do this you will have a true nightmare in managing too many expensive analog interfaces. Note that for 80 users, you will just need something like 10 ISDN channels, not much more. That will allow you further savings.
Note, even if telco will say you have to wait, not easy, etc, you must insist. The digital lines are available anywhere in the world.
After that, you can use an UC560 that has all and does everythingready, or an ISR G2 router with CME, for which you have to buy additional parts, licenses, etc. In both cases, you should have a reputable consultant or certifed Cisco partner, otherwise if you try to do it yourself you are likely to waste much time and make costly mistakes, and come out with a poor result for your customer.
