Direct SIP Dialing from FreePBX

Hi all, I wanted to see if someone can help me on something which used to work but now has stopped working.

In the past there would be times where I wanted to send a SIP call direct from one server to another. Essentially what I did was created the extension on both ends and then I simply when to the server where i planned to do the direct sip dialing. Go to the extension and then change the field under the extension from SIP/ to SIP/@

obviously no <> is included. so for example under the dial field would be SIP/3400 and i would change to SIP/[email protected] and lets say the server also had the same extension.

This used to work but now when i dial I get a 503 error and looking at the CLI it says

– Called SIP/[email protected]
– SIP/ is circuit-busy

Can anyone offer any advice on what would have changed. I checked the CLI on the other end to see if anything is coming through but I see nothing.

Thank you