Bingo! Thanks for the answer. I am already on a different port (ATA is on 5060, Zoiper on 5080). Just as instructed, using Windows Firewall, I blocked by IP all except vancouver.voip.ms server, which I am using. It seems to have worked because I no longer get calls if I register to a different server, but I DO hear them if I am on vancouver.voip.ms. Did not hear ghost calls for a couple days, they were not frequent, and now I am sure I won't hear them at all :-) I am behind a NAT (Sipura 2100 acting as a router as well as ATA.)