And how can it be configured as such? We don't need transcoding or anything like that, just something between our switch and the internet.
You need an IOS release with Multiservice support (now called Cisco Unified Border Element, formally Multiservice IP-to-IP Gateway).
If you go here:
You can click on Search By Feature, and put in Multiservice. It looks like you need at least IP+ or Enterprise and 12.4 to get that feature on the AS5350XM.