Can an employer do this?

This 'contract variation' happened a while ago and I didn't think too much about it until recently when they decided they wanted to implement on-call finally.

Iirc our team had a meeting where they laid out the plan for how on-call would work with the usual 'reach out if you have questions'. They followed it up with sending us an email with a copy of this letter and it seems like this was their way of finalizing it as that was the last we heard about it at the time.

I didn't have the mental energy to question it originally, but I'm not a big fan of working on-call seeing as that's not what I signed up for originally. My understanding is we have to agree to a variation in contract? Or is a lack of contest legally considered agreement?

Red is company and blue is our department for clarity.