iPhone disconnects call in Viber & Vonage on moving from wifi to 3g

My iPhone breaks the calls when i move from Wifi to 3G while speaking to someone on Viber or Vonage apps.

I want to know if there is any setting I can use to prevent this calls from disconnecting. Most of the time, the calls go blank and I have to dis-connect because there is not voice i can hear.

I know this is definitive iPhone issue, and was thinking if there is any alternative. Any alterbative like an app will also do. I keep talking to my wife in India like 3-4 hours per day, and its quite annoying when call goes blank.