Hi!
Have you tried another SIP phone or maybe a software one like MicroSIP or Zoiper to see if you have the same problem?
Another solution would be to monitor the SIP communication with a "sip set debug on"...
Did you try one of these phones that see "ghosts" like these on another system to see if they could be to blame?
I think it would kind of help if you knew where to look at, the FreePBX side of things or the SIP phone side of things...
Good luck and have a nice day!
Nick