The only fully plug & play supported gateways come from Audiocodes. There are different models, including the TP-260, which is a PCI card. sipXecs supports all Audiocodes gateways including FXS, FXO and T1/E1. Do not choose a gateway that offers both FXS and FXO ports on the same unit. Since these two port types are handled differently in sipXconfig, it is not a good choice.
We also have many users using Patton Electronics SmartNode gateways or Cisco Routers as gateways. There is no config support for these devices so they need to be manually configured.