Spyro,
Iit's not that easy to know exactly what happened. Who knows what the carrier(s) of that particular international call did to it prior to delivery? Generally speaking though, the calling number carried can be manipulated by a variety of parties carrying the call, is almost never necessary in order to route the call properly (but maybe for charging) and is obviously not necessarily the actual/real number belonging to the party who's calling you (assuming they even have a "real" e.164 number and are not just calling from skype or something like it).
In this case, my buest guess is the call was routed via the UK and the calling number was modified thus for reasons internal to some/all of the carriers involved (i.e. not necessarily a bug; they could be using it this way to settle charges between them or something like that). Another possibility is that your cousin was using a calling card platform located in the UK, etc etc.
Regards,
Dean