the SupraModem2400zi(plus or standard model) would fit the bill perfectly and add Modem connectivity to the computer... but is there something else that I should concider instead perhaps?
I have used nothing but US Robotics modems and I love them (although Im on broadband at the moment), they did the 56K messenger which worked when the computer was turned off, taking calls and all sorts.. it worked great with STFax too.
That said, how about you try and find the Amiga modem made for the A1200 surfer pack, gut it and stick a 56K modem inside, it will have an Amiga brand but be fast. (Bit like an Amiga One then ;-) )