Invalid remote extension

Hello! i’m new to the forum, so please be patient.

I have two Freepbx Servers (A and B), NO NAT between them. When I dial an extension from server A to Server B, I can comunicate, but if the extension is invalid, I get this response:

-- Got SIP response 503 "Service Unavailable" back from X.X.X.X:5060
-- SIP/ServerB-000099ea is circuit-busy

Then the caller from server A get a message that all circuits are busy, but what really is happening is that the extension he dialed does not exist on server B.

Is there any way to let callers on server A know, if they dial a wrong extension number on server B, that the number is wrong? instead of the “All circuits busy” message they are getting?

I have a SIP trunk between the servers. It works fine if the extension exist.
I have FreePBX, Asterisk

Any help will be really appreciated.