Even with a ported number, inbound call routing still heavily relies on the "number range" owner to direct the incoming call to the correct network.

If the original number range owner has their subscriber database go down, they can't do the lookup for the network to direct the incoming call towards, so it can cause disruption. The same is true if the incoming signalling endpoints are unavailable, as the incoming call requests won't be responded to.

Tis what I meant - I have an EE MVNO SIM but originally an O2 number and I can recieve calls just fine.