Still doesn't work after I forwarded port 5060 to ATA1 and 5061 to ATA2 (in the SIP Port setting of ATA2, I also tried to change 5060 to 5061 to match the forwarded port, still no help).
I then thought I might need more ports to forward to each ATA, so I forwarded both 5060 and 5061 to ATA1, and both 6060 and 6061 to ATA2, still no change.
Basically, both lines can make or receive calls at the same time no problem while sharing the same Internet Connection (even before I tried any changes to forward the ports), but when one FPL line calls the other FPL line, ATA 2 can never hear ATA1. There got be some workaround
For now, the only way to make calls between the 2 condo unit is to call the cell phones instead...