I'm having the same issue. It started after I upgraded software from iOS 7 to ios 7.0.3. When I opened my02, the "msisdn...code 36" message popped up. I've tried deleting and reinstalling and it hasn't work. Would like to try the fix mentioned above but though I've tried to locate the app in settings, so I can use the fix above, I Can't find it other than in "Usage" and the only option given at that point is "delete app". Any other fixes that may be suitable for iphone 5 would be ace.
The fix above won't work on an iPhone. You'll have to delete the app then do a soft reset before re installing. I'd download a fresh too rather than from a cloud backup.
I have no affiliation whatsoever with O2 or any subsidiary companies. Comments posted are entirely of my own opinion. This is not Customer Service so we are unable to help with account specific issues.