The Problem When making audio calls using SIP the phone rings but when it is answered there is only one way audio or no way audio. What Cause One Way Audio The cause of one way audio is a combination of NAT and STUN (which we’ll come onto later). Let’s talk about NAT first. NAT by default blocks ALL incoming connections from the Internet. This is well known and isn’t normally a problem; if you want a server accessible through the Internet you just port forward the relevent ports to it. The issue here is that SIP uses a large range of ports and it will choose one at random for each SIP call. We can’t just open our network up to a massive range of ports, it is bad security practice. More to the point how does it even work if you are not port forwarding any ports? All inbound traffic should be blocked by the NAT because there is no port forwarding going on. So how does it work? It works by using a technique known as UDP Hole punching.