User Busy Message

When I call one particular mobile number (either through WIFI or mobile network) I always get a USER BUSY message and engaged tone.
I have tried this number from a different mobile also on Smarty and get same problem. If I ring through my mobile VOIP service the call always connects - user is not busy.

The phones making the calls are iPhones and the number being called is also an iPhone.

All other calls proceed as expected.