SIP Extension Registering but failing to establish call


I have a PBX set up with a number of IAX extensions which work fine. However, when I try to use a SIP extension (in this case X-Lite 4) it registers with FreePBX but cannot connect a call. When you try to call out from the extension it says “[color=red]Failed to establish call[/color]” and when you try to call it from another extension or from outside you just get a busy/engaged tone. I’ve been unable to find a solution on the internet so far. Does anyone know what could be causing this?

I’m using the FreePBX Distro with FreePBX 12.0.1a16 and Asterisk 11.7.0. Nothing seems to change in the log files when I try to make a call.