The tricky part is that "mandatory" doesn't always mean what your manager thinks it means. Most employment law gives you the right to refuse work on scheduled days off, but your employer also has the right to discipline you for refusing - and that discipline can go all the way to termination. So technically they can't force you, but they can fire you for saying no. The catch is whether that firing would be legal or count as wrongful termination, which comes down to your exact location, your contract, and what your local labor laws protect.
Your contract matters hugely here. If it says nothing about on-call requirements or mandatory call-ins, you're probably in a better position to push back. Some places have laws that protect scheduled time off (certain European countries are strict about this), while others basically let employers do what they want unless there's a union agreement or explicit contract language protecting you. A few jurisdictions require paid time off to actually be guaranteed, while others treat it more like "we're scheduling you for this, but we reserve the right to change it."
Before you do anything, pull out your contract and look for language about scheduling, call-ins, or flexibility. Then check what labor laws actually apply where you work - not just what your manager says. If staffing shortages alone aren't considered an emergency in your area's labor code, regular call-ins might violate your rights. Document these calls too, in case you need proof later. If you're unsure, talking to your local labor board or an employment lawyer is worth it, since the stakes here are your job.