Aastra 480i Remote behind a Sonicwall Firewall

I have a user who works remotely behind a Sonicwall firewall. He is using an Aastra 480i. When the phone is first plugged in, it works fine. After about 10 minutes, all calls (whether inbound or outbound) take about 10 to 15 seconds before they start ringing. Any clue how to fix this?