Change the following in your sip.conf file:

externip = xxx.xxx.xxx.xxx ; This is your Public IP Address
localnet=192.168.0.0/255.255.0.0; Depends on your network class.
fromdomain=byo.engin.com.au; change from voice.mibroadband.com.au

1. occurs whether Asterisk -based Box is behind NAT or on a public IP address
2. only occurs on outbound calls to Landlines (Mobiles are fine) *
3. occurs everytime at exactly 20 sec mark without fail
4. all published trunk settings for Engin make no difference, including trying various other forum posted trunk settings for Engin

Thanks to Bob Fryer from Voice Integrity and Mr Mike for this solution.