Yes, You can do this
1. create local sip trunk in the /etc/asterisk/sip.conf
[local_sip]
type=peer
host=127.0.0.1
port=5060
context=my_trunk
dtmfmode=rfc2833
disallow=all
allow=alaw
2. create agi-conf5 then set "ivr_voucher=yes"
3. create dialplan in /etc/asterisk/extensions.conf
;assuming you need dial "8" to get voucher service
[my_trunk]
exten => 8,1,Agi(a2billing.php|5|voucher)
exten => 8,n,Hangup
4. create trunk "local_sip" in provider -> trunk
5. create prefix 8 in rate of dialplan and select trunk = local_sip
6. when system call you back and request destination from you, just press 8# you will get voucher prompt
Regards,
Nui (
Asterisk, A2Billing Thailand)