Why can I not make calls even though I can use data?

FIRST PLEASE MAKE SURE THAT DATA IS WORKING AND IF NOT SEE THE FAQS FOR GETTING THIS WORKING.

If you cannot make calls but can use data on your O2 or Vodafone plan please try the following:

1.  Even if you are calling a local number still make sure that you are using International Calling format e.g. If you are in Romania instead of dialling 075 861 88485 please try: +40 758 618 8485 (that is “+40” for Romania, then drop the leading “0”)

2. Try disabling VoLTE in your phone's settings.