HTC Touch HD and problems with international phone calls

Hi guys,

I have a problem with an HTC Touch HD (Vodafone) and international phone calls. The problem is that some people (usually from UK) cannot reach me at all(I even have no signs from voicemail!).
Strangely enough my girlfriends X1 is taking those calls - with her SIM card. Is it possible that I have some problem with settings in Windows or is it rather a problem with a carrier?

Any ideas are welcome.

Thanks in advance,
Mik

P.S.
Sorry, that I'm writing in English. I cannot speak Dutch or at least not that good to describe everything.

Dit is een Nederlands forum en daarom staan we ook alleen de Nederlandse taal toe op ons forum. Dit staat netjes in onze voorwaarden vermeld en daarom gaat dit topic dicht.