I just got a Sipura 3000 VOIP adapter. I've connected it behind my Cisco 831 router and am connecting to the Internet via Earthlink to Voicepulse.
I get a constant busy in this configuration. As an experiment, I connected the Sipura 3000 directly to the cablemodem and reobooted both of them (after powering down the cablemodem for more than thirty seconds). The unit worked. So, I put it back to normal and I get a busy signal every time I pull up that line.
I contacted Voicepulse. They told me that my Cisco 831 router was blocking the SIP on port 5060. I've installed both of these:
ip nat inside source static udp 10.10.10.232 5060 interface Ethernet15060 ip nat inside source static tcp 10.10.10.232 5060 interface Ethernet1 5060
I rebooted the router (after saving the changes to the startup config). It still doesn't work. I can ping the Sipura from my PC.