Call negotiation failures occur when a call cannot be successfully established. When this happens, the call will not connect at all. While Aircall implements measures to ensure as many calls as possible are successful, some circumstances are outside Aircall's control. These issues typically originate from local network hardware, firewalls, and their configuration.

This article outlines common network-related causes of call negotiation failures and how they affect Aircall.

Deep UDP restriction

Some networks, especially highly secure corporate networks, may prevent UDP packets from traversing through the NAT (Network Address Translator).

Aircall connections may transmit audio packets via TCP instead; however, those packets are still UDP framed by TCP. As a result, networks that perform deep packet analysis may still detect and restrict this traffic as UDP and block its transport through the NAT.

Allowing UDP transport for Aircall packets on the network should fix this problem.

SIP ALG

SIP ALG (Application Layer Gateway) is an extension of many NATs and firewalls, and is commonly found in both commercial and home router devices.

SIP ALG was originally designed to address shortcomings in SIP and VoIP packet handling. In practice, many implementations fail to manage this data correctly, resulting in:

  • Call quality deficiencies
  • Problems with call establishment

In Aircall, call connections and call quality are managed entirely by the application itself. Any use of SIP ALG can therefore hinder calls made using Aircall.

It is recommended to disable SIP ALG within your network, if this option is available.

VPNs

VPNs (Virtual Private Networks) provide a tunnel to another location so that network throughput to and from your device appears to originate from that remote location.

When used with Aircall, VPNs can:

  • Add latency, jitter, and packet loss to calls
  • Negatively impact call negotiation, as network addressing can become confused and call connections may miss their intended recipient

Aircall strongly advises against the use of VPNs when using the Aircall application.