When a call comes through to our network for your number, we send that call through to you via the internet. If your device is connected directly to the internet, for example a mobile phone on 3G (using a softphone such as Zoiper), the call will come directly to your device. The softphone will ring and you will be able to hear the person on the other side.
If however your device is connected to the internet through a modem/router (eg. via Wi-Fi or network cable), the call from our network is sent to your modem/router (which is providing the internet connection) and not to your device directly, as your device is actually on a private network sitting behind your modem/router.
In order for your VoIP device to receive calls and all the associated media (audio) for those call, your modem/router needs to know that it must send this traffic to the VoIP device on your network.
If your modem/router has not been setup to do this, it will try to "guess" where it should send the VoIP traffic to. This might work sometimes but is very erratic and you may find issues with registering your device on our network and/or only receiving one-way audio on calls (you cannot hear the person on the other end).
You need to setup a "rule" on your modem/router so that it knows it should send all VoIP traffic to your VoIP device.
This "rule" is known as Port Forwarding.
Almost every DSL/Fibre/LTE router has a Port Forwarding section built into it. You would need to check your router's manual to determine how to access this interface and how to enable port forwarding for your particular router.
You should also ensure that your VoIP device (whether that is a VoIP deskphone, computer with softphone or mobile device with softphone) has a static IP address on your network. For example: 10.0.0.5.
Once you have setup your VoIP device to use a static local IP address on your network and you have accessed your router's port forwarding interface, you then setup two port forwarding rules.
All traffic on ports 5060 (VoIP registration) and 10,000 to 20,000 (VoIP media/audio) should be forwarded to the LAN IP address of your VoIP device (eg. 10.0.0.5).
As all modems and routers are different you will need to check the user manual to find out how to setup port forwarding on your router. Note, on some routers this is called "Virtual Server".