FR DO call at 10pm the night before an early stby and DO call at 4am to advise you that you will be called out at 5. They DO call at 5am to advise that your original 9am duty has changed to a 6 report.
They do call at 5am on an off day to see if you're stupid enough to work a day off.
This isn't rumour, it's fact. 2 phones is the only answer.