Configuring an FXO port as a trunk

I have a zoom 5801f, which is connected via the fxo port to my analog line.
The box is configured as an extension, so that if an analog call is received, this automatically ‘hot dials’ a ring group.
If I dial extension 206, the zoom answers and connects me to the analog line giving dialtone.
I want to create a trunk so that I can dial out using the analog line.
I have tried using the custom trunk option, but putting
SIP/206$OUTNUM$ does not work (all circuits are busy).
The zoom does not answer 206 immediately so need about 2 seconds delay before sending $OUTNUM$ so put ww in the Outbound dial prefix.

Am I going about this the correct way i.e. creating a custom trunk, and if so what should I be putting into the Custom Dial String?

Asterisk 1.4.18
FreePBX 2.5RC3