I tried Viber once and I still went back to a plan. This one supposed to work on poor network environment but you still need a data plan. At this rate the telcos will unilaterally reduce our data from 12GB to 2GB to 1GB and still increase the price.
no matter what these *free* VOIP services offer, they still need some form of packet-data services as the backhaul, be it wifi or GPRS/3G/4G services to work. so Telcos will increase the price for data subscription and it is still LPPL.