If they aren’t in the contract, and don’t get paid at after-hours rates, ignoring them is always the right call. You’re either on call, and paid for it, or you aren’t. The alternative, is slavery.
On the other hand, if you work remote freelance, “after hours” becomes more of a nebulous concept, with calls and meetings being either scheduled or unscheduled. Unscheduled ones are fair to get ignored for up to 12 hours, with “send me an email” being also a good response.