STUN = Session Traversal Utilities for NAT

The internet has mostly two types of IP addresses, public and private.

At home, your phone, laptop, Alexa etc each has a Private IP address. Meanwhile, your router has one Public IP address.  The router uses a system known as NAT, which receives the data for all of your devices, and relays the traffic between the public address and each private address.

If each SIP device in a 2-way audio call is behind NAT, they each (and our SIP servers) need to know the public IP address of the other, along with the allocated port which NAT has assigned to relay the traffic.

Depending on various factors, a SIP client might not always know a router's public IP address and associated port for a call. sip.audio provides STUN servers which can help to decipher and pass on the required credentials, so long as the NAT configuration supports this method.

So, adding stun.sip.audio to your SIP device configuration will help to ensure successful calls in many (but not all) cases. If in doubt, we advise enabling STUN by default, and to only try disabling it if calls are failing.