Vodafone India — Roaming Charges or I think unethical Charging of money!!!

Address:

Gautam Buddh Nagar, Uttar Pradesh

I have a vodafone postpaid connection. I have opted for roaming incoming free pack from Vodafone. When I am at my hometown in Agra( in roaming) the received call are maximum times on Airtel Network but outgoing is always on Vodafone. I lodged a complaint with them and they are ready to refund my money as good gesture but I want to know only one thing. How is it possible that all the calls are received on some other network(in my case airtel) and all outgoing on the same network(vodafone). Even is network is on Automatic and not manual there should atleast one instance that the outgoing is also on some other network. The number of call i make is always high so there is very less chance of misunderstanding.