I reckon it is a problem with your International Roaming.
This should help.
Read: Managing your international calls.
My mum has iPhone 4s with iOS 5.1
She is also on orange (although phone was bought sim free). She has just gone to tenerife and same thing has happened to her. She tried her sim in my aunties phone and it worked but my Aunty got "no service" with my mums Phone.
This scenario seems to rule out international settings and no available network from the issue. Any help kindly received!!