Thanks to Kerry from SOS4Biz for highlighting an issue that seems to arise on some UK PRI lines with the Positron G320 and V310 Telephony products.

If you try and make an outbound call over the PRI lines and recieve a busy conjested message then please add the following to "/etc/asterisk/zapata.conf"

;UK
pridialplan=unknown
prilocaldialplan=unknown
overlapdial=yes
;END-UK

Positron will shortly be adding these changes to a new firmware.