It depends on the nature of your work. In some types of work, individuals are supposed to do their own time management, and carve out chunks of time for activities that require sustained concentration, and respond appropriately to occasional requests, which are a mixture of low- and high-urgency. For example, someone who needs to read complex academic papers, but occasionally might be interrupted for an urgent produc…
Async comms should regularly (and, perhaps, by default) be muted to enhance focus. Let them collect and allow the person to manage their own time. "No hello" allows them to have the question ready for them to address when they process their incoming queues.
Synchronous comms should be used to get immediate attention on an urgent issue that needs to be addressed immediately, or be used for tight, rapid iteration in a discussion (e.g. rapid design discussions). Because it necessarily takes attention away from another task, the activation energy should be higher.