If you want to have a2billing users in the fop2 display, you can add the following snippet at the end of the autoconfig-button-freepbx.sh script or use this as your own autoconfig script.. you will need to modify the mysql credentials if they are different:

mysql -EB -u a2billinguser -pa2billing mya2billing -e "select concat('SIP/',name) as channel, if(firstname<>'',concat(firstname,' ',lastname),name) as label, \"extension\" as type, \"a2billing\" as \`group\`  from cc_sip_buddies left join cc_card on name=cc_card.username" | sed '/\*\*/d' | sed 's/: /=/g' | sed '/.*=$/d' | while read LINEA
do
echo $LINEA | sed '/NULL/d' | sed 's/^channel=\(.*\)/\n[\1]/g' | sed 's/^extrachannel/channel/g'
done