Can't do Dial() with a FQDN

I set up an Asterisk at Home with only a few registered SIP users. There are no ZAP, IAX and PRI lines.

While I can dial a full FQDN (e.g. on the X-Lite to reach the other SIP user, I can’t do the same thing with dial() in the dialing plan. It gives me a “All Circuits are busy” message.

Is it possible to use FQDN in the dialing plan? How to do that? Thanks!