I've had good success with the UltraHub behind another router acting just as an ATA, but I don't think your Asus will be capable enough to do this - you need a device with more advanced LAN configuration options.
You could consider porting your number to a 3rd party VoIP service like 2Talk and just going for a naked connection with Vodafone and a 3rd party ATA...all depends how complex you want to get.
You could also consider changing providers to one that uses the ONT for voice, like Spark.
