I've had VOIPo service for a few years (and have renewed through 2014). Mine adapter sits behind a Tomato-based firewall and is flawless. Never get one-way audio, always have a dial tone.
I also have viop.ms service and it also is flawless behind a firewall.
SIP ALG needs to be disabled!
Sounds like a configuration issue, and if an issue exists using VOIPo, it will probably exist with any VoIP provider.