Asynchronous, meaning the sending of an email and the reply
Just by the nature of sending an email, we’re saying “get back to me as soon as you can.” If you need an answer right this second, people don’t email. They use a communication method that will trigger an instant notification, like a phone call or text. People generally don’t allow random incoming emails to trigger notifications¹.
If you need an answer from someone immediately, and you don’t have any immediate communication methods in common with them, you might just be out of luck. Perhaps write with a sense of urgency and expectation in the email, and try to establish another communication method as soon as you can. For example, I’ve hired a babysitter and lined up everything so far via email, it’s on me to make sure I have a more immediate communication method than that.
If you don’t have someone’s phone number, there is a chance that’s because they don’t want you to have it.
This asynchronous nature of email, and how it’s become the expectation, is another reason for the success of email. It makes email a lot more comfortable. It allows people to be a lot more public about sharing their email address, which is something we’ll have to talk about soon.
¹ Although! I had a fellow from Sweden once write to me to say that he doesn’t like how the weekly email newsletter from my product arrived in the wee hours of the morning. He keeps his phone by his bedside and the beep from the incoming email wakes him up. I really wanted to write some custom code just for this one guy to make sure that didn’t happen to him, but I never did get around to it.