Advanced - QoS/Security (Tab) - Security(Section)__Filter packets from Registrar
If set to “on”, all SIP packets not coming from the registrar/proxy will be ignored. For security reasons, “on” is the default setting. This may cause big problems in an environment where SIP packets from other sources also have to be accepted for proper functionality! You have to disable it to make a call flow work which isn't going via the proxy only !
Note: Even in case you set Registrar or Outbound Proxy to an FQDN that resolves in multiple IPs, only the IP used to send the REGISTER to will match