Many modern routers are becoming clever at detecting the sort of traffic VoIP sends and allows it in but some others don't.
Some have a process running called SIP ALG which is supposed to help your router handle VoIP traffic but generally it just gets into a tangle with it. If it's switched on in your router, please turn it off.
If you're still having problems, most VoIP service providers have a service called 'NAT Proxy' which sorts out problems of one way calls.
If you experience one way audio or you are able to make outbound calls but not receive inbound, just enter your VoIP providers NAT proxy address as your Outbound Proxy. It will look something like this: nat.voipfone.co.uk:5065
Adding the NAT Proxy in a telephone or adapter
To do this you will need to enter the IP address of your phone into the web browser of a PC that your telephone shares a network with and find the Outbound Proxy field. If you don't know how to do this please consult the manual for your telephone or adapter, otherwise ask us and we will try to help.
Adding the NAT Proxy in the X-Lite Softphone
To do this in X-Lite softphone press F8 when the phone is open and you will see the menu. Add the NAT address of your VoIP service provider into the outbound proxy field shown there.
Note: Please try your phone without the outbound proxy first; most equipment will not need it and adding it unnecessarily may cause problems. If you have the proxy added and experience problems please remove it & often it does the opposite of what you would expect!