[galdosdi]

That's just fine, as long as on-call expectations are fixed as part of the job offer, just like salary is. If the on-call expectations change for the worse later, that is exactly the same as a salary cut.

IANAL but I believe (in the US or in NYS at least) this would be grounds for quitting with full unemployment coverage (you are not usually eligible for unemployment benefits if you quit, exceptions include the basic nature/hours/wage of the work being changed without your consent, since that is philosophically similar to having your old job terminated and being offered a new, worse one)

Get any on call expectations in writing when taking an offer if you can, just like you'd expect salary in writing