A big drawback to the previous post is that ATAs cannot be used for fax machines. The reason for this is because the gateway, when sensing a fax tone, immediately moves into T.38 mode. The ATA is unable to handle T.38 and so tries to continue in g.711 mode. Since these two are incompatible, the call will fail.
However, even Cisco recommends that a fax machine be connected to a VG224/VG248 or FXS card connected to a router. These devices support more than just straight passthrough for fax machines.