Enabling this sends a SIP Options packet is sent to the SIP Provider every 60 seconds. Depending on your router, this may help to keep the NAT entries active and your SIP device won't be moved to another port due to (SIP) inactivity. It's also can be used to determine the latency of the far end, response latency for these requests are shown in the System Status page.
To enable or disable:
Go Setup > Voip Providers
Click Modify Voip Provider
Select Connection Settings
Set Qualify Hosts to YES or NO to enable/disable
Note: Do not enable this option if your provider does not respond to SIP Options packets.