I know, there are still different communication requirements for different airspaces.

When you file your travel plans you'll have to check in with different ATCs at different points, you are also likely to be contacted by ATCs depending on the flight plan, conditions and various other factors during the flight.

If you do not check in on schedule e.g. when there is a handover of ATCs or you do not respond to ATCs that trying to make contact with you, there will be a flag and an alert will be likely sent. If this goes on for minutes then they'll assume something was wrong and follow their procedure to make contact and investigate which might involve scrambling jets to make visual contact.

It's not like pilots can be too busy to respond to ATCs yes they might delay a response if they are in a middle of a sentence with a member of the air crew but that's a 5 seconds delay. Even a 1 minute delay would be flagged and you'll likely to be asked why you haven't responded or made contact on time.