npub1hvzjvzfmclu5l5zk65fxye5cgt7e0qvs6rcn60z72v9hy79mmpzq4qmle9 (npub1hvz…mle9) In British usage, we generally say "off sick" for taking a day off due to illness.
"On Sick" (or more often "On the sick") means that you're either permanently or long-term ill, disabled or otherwise not able to work.
"Calling in sick" is the actual act of ringing your boss up (or nowadays, emailing them) and telling him you're not going to be in work today