I believe this is perfectly legal... Your duty time starts at 17:55. Since you haven't actually reported in yet, they're free to change your schedule as they see fit... As far as the company is concerned, they're doing you favour.. you get to sleep in!!! ;-)
I don't think this is the correct way to treat your staff though... Aren't there rules in place that prevent your company from keeping you on a leash like that? Terms & Conditions, anyone?
I've been in more or less the same situation... I resolved to not pick up my (work-)phone prior to actual reporting time. I'm theirs between punch in and punch out, the rest of the time is mine... MUHAHAHAA :-)