The most common answer to this is that your network doesn't allow VoIP calls.
First, follow this article on how to configure your network. If that doesn't solve the problem, you'll find a solution in the following page.
You might want to change the transport protocol of your app.
How to do it on the Mac app
How to do it on the Windows app
The network administrator may have blocked VoIP traffic. In that case, the central Phone icon in your Aircall app will remain red.
Ask your network administrator to unlock the following ports:
SIP and WebRTC Signalling (SIP Websocket)
IPs : 126.96.36.199
TCP ports : 5060 5061 5080 5063
UDP ports : 5060 5061 5080 5063
Media / RTP
UDP ports : from 16384 to 32768
Alternatively, you can set-up a VPN to connect to a distinct network that doesn't have these restrictions. Contact us on firstname.lastname@example.org if you need help on this.
There's nothing we can do really here. We do not believe hacking shared WiFi networks to make calls is good :-)
If you're not in one of the 3 cases above, try restarting your App and if you're a Mac user, check if the problem happens both on web and Mac app.