Failure to negotiate a call connection will prevent the call from connecting. While Aircall implements measures to ensure as many calls as possible are successful, some circumstances are outside of Aircall's control, with such issues typically originating from local network hardware, firewalls, and their configuration.

Deep UDP Restriction

Some networks, especially highly-secure corporate networks, may prevent UDP packets from traversing through the NAT (Network Address Translator). While Aircall connections may transmit audio packets via TCP, instead, those packets are merely UDP framed by TCP. As such, networks that perform deep packet analysis may still restrict their transport through the NAT.

Whitelisting UDP transport for Aircall packets should fix the problem.

SIP ALG

SIP ALG (Application Layer Gateway) is an extension of many NAT’s (Network Address Translators) and Firewalls and are commonly found in commercial and home-based router devices. The intention of SIP ALG was to address shortcomings in SIP / VoIP packet handling, but many fail to correctly manage such data, resulting in numerous call quality deficiencies and, as importantly, call establishment.

Call connections and call quality within the Aircall app is completely managed from the application itself. As such, any use of SIP ALG can only hinder calls made using Aircall. It is, therefore, recommended to disable SIP ALG within your network, if available.

VPNs

VPN’s (Virtual Private Networks) provide a tunnel to another location so that throughput to and from your device appear to originate from another location. Aside from adding potential latency, jitter, and packet loss to your Aircall call’s, using a VPN may also negatively impact call negotiation, as the network addressing can become confused, resulting in call connections missing their intended recipient.

Aircall strongly advises against the use of VPN’s when using the Aircall application.

Still have questions? Please feel free to reach out to the Quality Engineering Team for help.

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