If you are attempting to do this on the cheap, a Linux box with multi-modem cards will allow you to perform this, but it doesn't necessarily integrate well with your Windows boxes (assuming that you are using them). Linux can be configured to work with Windows, but if you aren't familiar with Linux, it can be a daunting task.
You are probably better off buying a Cisco Access Server or Ascend Pipeline like jimbopalmer recommends. They are extremely reliable and integrate easily into your network.
But be prepared to have brute force password hacks on these servers with frightening frequency. 800 numbers generally receive a large number of hits from war dialers.
Also, if you use analog lines for your modems (rather than the modem emulators in jimbopalmer's solutions) you will not receive a 56K connection. 33.6K will be your best theoretical speed, and speeds around 26K - 30K will probably be the norm.
I have to agree that nationwide dialup accounts from an ISP would probably be more cost-effective. Especially if you count the travelling personnel's time as less than free.
pansophic
