Canot dial out on new phone

I have just bought a new Samsung galaxy express GT-I8730 mobile. I have done the usual starters like inserting some numbers of my friends. I have also filled out the Samsung account details which were accepted. I can receive calls OK but when I try to dial an outside number, I get "Not registered on Network". I have read the downloaded manual on my computer and the help file on the phone and also read a book I bought on androids. I have also signed up with my service provider. Nowhere have I seen anything like registering on a network. I probably have overlooked something but cannot find where. Can anybody help?