Try this:
ACTIVE GW
=========
!
interface Ethernet0/0
ip address 10.6.0.21 255.255.0.0
ip helper-address 10.5.0.1
no ip route-cache
no ip mroute-cache
half-duplex
standby timers 3 7
standby priority 200 preempt
standby ip 10.6.0.199
standby track Se0/0 101
h323-gateway voip interface
h323-gateway voip id gk-2 ipaddr 10.16.19.2 1719
h323-gateway voip h323-id 3640-1-A
h323-gateway voip tech-prefix 2#
h323-gateway voip bind srcaddr 10.6.0.21
!
STANDBY GW
===========
!
interface FastEthernet0/0
ip address 10.6.0.20 255.255.0.0
ip helper-address 10.5.0.1
duplex auto
speed auto
standby priority 100 preempt
standby ip 1.6.0.199
h323-gateway voip interface
h323-gateway voip id gk-2 ipaddr 10.16.19.2 1719
h323-gateway voip h323-id 3640-1-B
h323-gateway voip tech-prefix 2#
h323-gateway voip bind srcaddr 10.6.0.20