You can't pick up, so it goes to voicemail. If they don't leave a message, you can't tell who called or whether they tried someone else.
That night, after dinner, you sit down to return calls, write up the quotes you promised and send last week's invoices. Some of it gets done. Some of it waits for the weekend.
Nobody's doing anything wrong. You're doing three jobs at once. But every missed call and every quote nobody chased was work you'd already earned a shot at.