Yes there is someone that makes a router that should be able to be used for a dialup connection.
I have an Asante FR3004LC Broadband ethernet router with a 10/100 switch and a built in printer server. It also has a built in COM port for an external modem. With the supplied software, you can access a printer attached to the router from any computer in the network.
And the manual indicates that if the Broadband Internet connection goes down, the router is suppose to fail over to an external modem for a dial-up connection. I dont have an external modem, so I haven't tried the dial-up abilities. But I presume that you could initiate a dialup connection from any of the computers on the router. Or at least if you get one started, all the computers on the router should be able to share it. But the more computers sharing a dial-up connection, the SLOOOOOWWWEEEERRRRR it will be.
Ebay has 2 of these routers for sale as I write this. But I'm not sure they make them anymore. I couldn't find them on the Asante.com website, only on Ebay. But I like mine. Having the built in print server is nice.