I've had some issues setting up a combination of a Draytek 2600G Router and Cisco ATA 188 to work with PlusTalk.
I have found that in addition to UDP port 5060 to support SIP traffic I also need to redirect (forward) UDP port 16384 to the ATA IP address to enable inbound voice traffic.
This port is part of a range 16384 to 32767 that may be used for media traffic.
Without 16384 redirected I could make out bound calls from the ATA sucessfully but on inbound calls I did not receive the inbound voice stream, even though the call alerted and connected and I could be heard at the caller end.
With UDP port 16384 redirected to the ATA everything works fine