that have modems built in are exclusively for use with broadband services. These routers contain ADSL modems that only work with a digital service, and your dial up connection requires the use of an analogue modem.
You can share the connection, but if you want to do it via a router it can get complicated, and to be honest the best way to run a wireless dial up network with only two or three computers is to use wireless network adapters and run them in ad-hoc mode. This means that at least one of the machines has to be on, and connected to the internet via a modem (internal or external) for the other computers to have internet access.
Frankly, the internet access will be very slow, and if two computers are surfing the net at the same time it will be like wading through treacle.