freePBX(Trixbox) not giving busy tone

I have an odd problem that I just cannot work out.

I am using FreePBX 2.2.0 (Trixbox 2.0) on a system with an FXO and an FXS port on a TDM 400 card.

I currently have all inbound routes (from PSTN and VSPs) directed to the analogue extension on the FXS port. When the phone is busy, any phone call coming in from my VSPs’ SIP trunk or from another SIP softphone will hear a busy tone. However, when the call comes in from the PSTN via the FXO port, it always gets a ringing tone. When the analogue extension is no longer busy, it then rings the extension where the called party can take the call from the PSTN line.

It seems when a call comes in from the PSTN it gets parked with a ringing tone until the extension becomes available. When I change the inbound route to always send “busy”, rather than direct it to the FXS extension the caller always hears a busy tone. so the system can send a busy tone OK.

What am I doing wrong?