To be honest, there doesn't seem to be a solid answer for you. The telecoms in Oman generally have been lobbying against Voip services, probably because they fear loosing lucrative income generated by international calls in Oman. There have been reports that Facetime has worked there within the country, but not internationaly.
Also that some ISP providers which are associated with phone carriers have allowed it, but stopped.
From what I've been able to gather, for all practical purposes Facetime is blocked in Oman, but this situation is still in a state of flux. In other words, don't expect Facetime to work there.