It's annoyingly a bit complex. As I now understand it, for Three in the UK the rules are pretty simple for most cases: UK user in UK calls UK number – local co

It's annoyingly a bit complex. As I now understand it, for Three in the UK the rules are pretty simple for most cases: UK user in UK calls UK number – local country cost. UK user in Germany calls UK number - same cost as calling locally from UK. UK user in Germany calls German number – same cost as calling a UK number. The weird edge-case is that calling a foreign number is less expensive when roaming; calling a German number from e.g. France is now cheaper than calling a German number from inside the UK, because the roaming regs affect roaming cost but do not affect what providers can charge for international calls from the home country.