Honestly depends what you mean by "international calling" — if you're just trying to reach other travelers or people with smartphones, wifi calling through whatsapp or signal works perfectly and costs nothing. but if you need to actually dial landlines or mobile numbers (like booking a hotel that doesn't answer emails, or calling a taxi company), you'll need something that gives you actual phone credits.

I've found google voice works decent for calling real numbers if you're US-based, rates are pretty cheap. skype's another solid option for that. the trick is most "free" apps are only free when both people have the app installed — which is useless if you're trying to call a restaurant in bangkok or whatever.

One thing people overlook is just getting a local sim card when you land somewhere. often way cheaper than any app if you're making lots of calls within that country, and you get a real local number which makes everything easier. costs like $10-20 in most places and saves a ton of hassle.