Confirm that incoming calls work ok, Stage Method is set to 1, and Dial Plan permits and does not alter the called number (the default should be fine).
The 10 second delay before the 503 leads me to suspect that Wait for Dial Tone is timing out.
If you have this set to Yes, report whether setting it to No changes the behavior.
Also, report what the Status page shows for Hook when idle and when attempting to call.
What country are you in? What is the 503 connected to (copper pair from central office, cable MTA, fiber ONT, etc.)?
To confirm that nothing is wrong at the PBX end, at the Asterisk command prompt type
sip set debug on
make a failing test call, paste the Asterisk log (which will now include a SIP trace) at pastebin.freepbx.org and post the link here.
Also, try connecting a line-powered corded phone in place of the HT503, using the same cord. Confirm that you can call out ok.
If you still have trouble, post screenshots of the FXO page.