I have also seen this behavior, but caused by the router on the far-end, but still a firewall issue.
If this is happening on phones at just one site but other sites are fine, check the router on the far-end, depending on the model/configuration it might be as simple as just turning off SPI.
If multiple locations are effected, it is clearly something on the Teleworker end. Although I have gotten this to work in DMZ mode, clearly the most effective, efficient, and least troublesome is in in Gateway mode with one NIC directly on the public network.