No audio or one-way audio is nearly always down to a NAT traversal issue. There doesn't seem to be a 'one solution fits all' approach either (apart from not using a NAT router, which isn't practical). You should try a combination of settings of STUN/port forwarding etc. Don't forget after each change, you should reboot the VoIP device to ensure it re-registers with the server.
Personally, I use Sipgate for my inbound DID and have very few problems; occasionally on a PSTN to Voxalot call (Sipgate as a registered provider), the calling party hears ringing once or twice and then gets Voxalot voicemail; the called party just keeps ringing. I've not had time to dig deeper in to the cause of this.
I also use a re-flashed Linksys WRT54GS wireless router that has a Sipath (SIP at Home web interface for the SIP Express Router) module loaded. The firmware is freely available at
www.dd-wrt.com. This is very good at overcoming any NAT traversal issues as the router acts as the SIP proxy and controls the mapping of WAN IP address/ports to LAN IP address/port. If anyone has compatible hardware then I'd recommend they give it a try (DISCLAIMER: you will need to re-flash your router - there is always a chance that you will kill it. BE AWARE. Having said that, if you follow the instructions then you shouldn't have a problem
)