Grandstream phones receive 'ghost' calls periodically (when answered, there is silence). Can I prevent this?

This is a well-known issue with Grandstream phones. They have a feature called Direct IP call. This feature allows to place direct calls to the phone without a SIP server, thus these calls are bypassing the switch and you cannot find them in the logs.

To prevent these calls, disable direct IP calling in the phone settings. Usually this option is under the Settings menu, Call features section.

If you use auto provisioning this feature will be disabled by default.