Hi I had a similar issue earlier this year. I couldn't make calls to the UK from spain or France despite being a pay monthly customer. I could dial numbers from my phone book but no others.
After having a VERY deep and meaningful conversation with someone from C.S whilst over in France it transpired that although I had no calling bars in place there was a deep and hidden call bar that can be placed on your account when you take it out that prevents you making international calls from your home country.
Apparantly with the new rules in place re calling and txt from EU zone countries that call bar can prevent you from making certain calls whilst outside of the U.K in the Euro Zone.
Interestingly this didn't happen with the old travel bolt on as I never experienced it until after the change in rules.
It may be best to get onto C.S and ask them to ensure there are no call bars of any description on there including the bar preventing you making international calls.
Mine was sorted in the end and I got a £10 credit on my airtime bill for the inconvenience.
Hope this helps
