I ported my number from Vodafone just before Christmas 2016. It took me a while to find out that certain people cant get through to me. Only common thing between them is they are all Vodafone customers . It is either a porting issue or a routing issue. But no one seems to know what I am talking about when I ring Optus...
My understanding is this is a problem on Voda's end however now that you're an Optus customer, you need to report it to them and then they'll sort it out with Vodafone.
I'd request they lodge a fault ticket for you and have them provide you with the reference number.