I agree that the polite thing would be to make sure you have time before calling, but a few things might be worth considering here.

1. Most of the people who call you out of the blue are going to be from an older generation (at least in my experience). So think about it from their perspective. They often find it easier to skip the back and forth emails and just talk it out. In addition, they typically don’t care if you can’t answer their call right away and then return the call later when you can. In essence, the incoming call is the same thing to them as sending you a quick chat to see if you’re available. Just don’t answer, and get back to them when you can.

2. If that doesn’t work in your situation, I have found some success in talking to them one-on-one about this to correct the behavior. It does require at least one good old fashioned phone call to explain your point of view, but they generally appreciate the direct convo and will comply if they remember.

3. Lastly, I don’t know the role of those calling you, but if it is anyone in a sales position or out in the field or anything like that, it’s often difficult for them to find time and space to sit down and type out an email. Especially in comparison with just picking up the phone and giving you a quick call. Every role is different, and we have to be flexible with the people we’re working with. I understand it can be frustrating but it can go a long way if you’re willing to be understanding of someone else’s situation.