Gergory Lee of Borderless Hub Pte Ltd was very helpful in sharing the following setting for the Elastix PABX. These settings are for the pfingo service by StarHub. The phone numbers usually start with "3" instead of "6".

Here are the setting.

Truck Name: "your pfingo phone number"

Outbound Caller ID: "your pfingo phone number"

Truck Name: "your pfingo phone number"

PEER DETAILS:
type=friend
host=sip.pfingo.com
fromdomain=sip.pfingo.com
username=userid
fromuser=userid
secret=password
insecure=port,invite
nat=yes
qualify=yes
dtmfmode=auto
disallow=all
allow=ulaw
context=from-trunk

Register String:
userid@sip.pfingo.com:password:userid@sip.pfingo.com:5060/the_extension_number_that_need_to ring