For some length of time, I’ve had problems with my Nexus One and the Facebook for Android application. Phone numbers from Facebook are imported in to the Android Contacts application with a country code but without a leading +, for example – 447700952155 rather than +447700952155. This makes them rather difficult to dial, as your mobile carrier won’t recognise them.
If you have this problem, try http://www.253below.com/prefixer (or search for Prefixer on the Android Market – find the 253 Below application). Set up a rule to match 44[0-9]+ and tell it to add a prefix of + to the number. Hey presto, you can now dial successfully.