If you are simply extending analog sets, you could use a FXO gateway -> network(sip) -> Asterisk -> Network(sip) -> FXS gateway.
On the cheaper side, Grandstream makes a 24 port FXS box that is nothing fancy, but works pretty well for a basic analog set. (I don;t know if you have 24 simultaneous calls, because I have never needed it.) grandstream.com/index.php/pr … ys/gxw4024
They do not make a 24 port FXO device though. You could wire in 3 - 8 ports through a patch panel.
grandstream.com/index.php/pr … ys/gxw410x
Or a mix, the 24 port Digium card with FXO ports installed into the Asterisk server and the 24 port FXS gateway on the far end.
These are just examples of some devices, there are similar devices from other vendors that you may like better. I have used the Gxw4024 because it is inexpensive and works for my non-critical phones.