I have already set it in my FreeSwitch\conf\sip_profiles\external.xml:

 

<gateways>
  <gateway name="FreeSwitch">
           <param name="username" value="1008"/>
           <param name="realm" value="218.210.xxx.xxx"/>
           <param name="password" value="1234"/>
           <param name="extension" value="1008"/>
           <param name="proxy" value="218.210.xxx.xxx"/>
           <param name="expire-seconds" value="3600"/>
           <param name="retry-seconds" value="10"/>
   </gateway>
 </gateways>

But my FS_A always return a 403 to FS_B, Where is the problem??