Hi prabeen,
buddy i think without flexi abis u can also activate ondemand edge ( 64 kbps ) timeslot by defining switchable timeslot. already we are working this configuration.
for example if i define numreq for chgr 1 – 8 and out of 8 i am defining 4 fpdch + 1 spdch .
then remaining 3 will work as switchable for egde and it can also use for voice when no egprs traffic.