If you have not already done so, try the recommendation of carlosalbffgomes given
above. If it still doesn't work properly then I would recommend that you try pointing ipkall and Gizmo5 calls directly at your ATA's SIP address, bypassing Voxlot. By doing that you will loose Voxalot's voice mail and other features for inbound calls coming from Gizmo5 and ipkall, but if you gain two-way audio then you will be better off than you are now. If you decide to receive calls directly to the ATA's SIP address you will need to configure the router to forward the SIP ports and RTP port range to the ATA.