Which cellular carriers worldwide allow FaceTime calls over 3G/4G Networks?
Standard, Apple doesn't allow you to make FaceTime calls on a 3G network. And however 4G is often faster then WiFi, the LTE network isn't supported either.

There are however some ways you can enable FaceTime over 3G using Cydia (so jailbreak) or you can use your personal hotspot feature (jailbreak too) to enable this.

UPDATE:
I've tested the method of Redmondpie (with iFile) and with a Belgium Provider (Proximus) it works like a charm!