I had the same problem on my 820 initially, i too use vodafone, i realised the call drops were caused when the phone's highest connection speed was set to 3G, i used headphones and observed the changes in network strength, the 3G would suddenly drop the strength & call dropped, phone would switch back to 2G.
So i now keep my highest connection speed to 2G, and no call drops ever!!(until the other side was on mtnl mobile network)
& improved battery life on standby too.
Also my old symbian phones all worked with Vodafone 3G all the time and never a call was dropped! It seems Lumia's can't work with as low strengths as symbian phones worked. <Sigh>
Also i really miss the option of locking the network on 3G only, like i used to do in my symbian devices.
I purchased Lumia 620 on 27th mar and from very next day i observed that Calls are getting dropped very frequently. Talked to the shop keeper and he suggests me to go for Nokia care. How fair is it that a brand new phone should be taken to servicing centre for no misuse on my part. I have also checked if there are any software updates available, but it shows your phone is up to date.
I am on Vodafone n/w
Is anyone else facing problem like this?