gfrench changing sim card is the easiest and most sensible solution. Only trouble is some of these leeches have ingenious ways of finding out your details and contacting you again. Follow Brigadier's advice and under no circumstances should you answer the security questions. You can turn the tables by asking them the same questions in reverse. Use the Foxtrot Oscar phrase as much as you like if they don't get off the phone and tell them exactly what you think of them. If the debt is SB then there is absolutely no obligation on you to contact them in any shape of form.