We've got at least 5 examples where SIM2SIM isn't working for calls at all. It's a carrier problem outside our network. Mobile data is working. It's a known problem, and it's being investigated.
We don't have an estimated time for a fix yet, but we'll post updates as soon as we can.
Apologies for the late update! This was fixed around 17:30 yesterday. It was some sort of signalling problem, and alternative routing was used as a workaround to get it up and running again.