ATA Configuration: 

default route = router IP (not PBX) 
Outbound Proxy=PBX IP:port
No dialplan needed in ATA - (set in Asterisk) 
SIP proxy = PBX IP 
SIP Registrar = PBX IP 
Register Expired = 180 
Authentication ID =  (PBX Username or Ext)

Endpoint Type: SIP Phone
Config Mode: Config by Line (even if only using 1 line. Would not work for me otherwise)

Asterisk config (sip.conf): 

[999]
username=999
secret=password
type=friend
description=ATA
host=dynamic
port=5060
dtmfmode=rfc2833
call-limit=2
canreinvite=no