Yes. Routers have multiple interfaces. The AS series basically uses T-1 or similar circuits as interfaces.
If by "modem" you mean that you would connect modems to it, then no. I don't think it can do that beyond the normal aux port. You get one or more PRI interface cards and install them. Each can support 2 PRI's, which can each support 24 dialup connections. Still speaking for the 5200, but I think the 5300 is extremely similar.
The 5200 has a single ethernet interface and two serial ones. If you're not setting up a dial-in service of some kind, then just get a router. The AS series can do it, but don't spend the money if you don't need all it's capabilities.