Prevent calls that are not coming from the SIP proxy.

This prevents calls that are often referred to as ghost calls.

resources/templates/provision/grandstream/grp261x/{$mac}.xml

@@ -370,7 +370,7 @@
 
 		<!-- Accept Incoming SIP from Proxy Only  -->
 		<!-- Yes, No -->
-		<item name="account.{$line}.sip.validate.incomingServer">No</item>
+		<item name="account.{$line}.sip.validate.incomingServer">Yes</item>
 
 		<!-- Authenticate Incoming INVITE -->
 		<!-- Yes, No -->
@@ -3036,4 +3036,4 @@
 			<item name="pks.ext.{$row.device_key_id}.value">{$row.device_key_value}</item>
 		{/for}
 	</config>
-</gs_provision>
+</gs_provision>