Using a SIP service and hearing an incoming calls' ring, i have no problem at all except both sides of incoming calls can't hear each other.
Have look in the web management pages for your router and look for something along the lines of SIP ALG (SIP Application Layer Gateway).
When you find it, turn it off as this interferes with SIP calls and is not needed. The classic problem it causes is the lack of audio so you maybe suffering from this.
You may care to google for this using the words "SIP" "ALG" and the brand name and model number of your router to find the correct page on the router where this is controlled.