@Moaningagainn This is inappropriate use of your emergency contact information, breach of good data‑handling practice and potentially causes your parents upset and inconvenience unnecessarily.
I would email your boss (so you have a record), bcc'ing yourself and set a boundary by saying something along the lines of "Please do not contact my emergency contact unless there is an actual emergency. For rota changes or shift swaps, leave a voicemail or send a text to my personal number as provided and I will respond when I’m available.”
Emergency contacts exist for the purpose of medical emergencies, accidents and situations where the employee cannot communicate. They are not a secondary switchboard for shift swaps, routine rota changes, "Can you come in tomorrow?”, calls on days off or anything that is not an actual emergency.
When an employer uses an emergency contact for non‑emergencies, they are potentially breaching normal HR practice, potentially breaching data protection expectations, creating unnecessary distress for the family, undermining the employee’s autonomy, and in your case, it’s especially intrusive because you are divorced, your parents may be your only support system, and now they’re being dragged into workplace admin and potentially worried you are missing!
If you are not answering, the correct process is to leave a voicemail, send a text or an email and try again later and in so doing, acknowledging you are not on duty, you are not being paid to be on-call so your boss is acting inappropriately.
If the boss does it again after this, then go to HR.