call failing in iphone 4s with ios 5.1

Call failing on iphone 4/4s with ios 5.1 from different locations and with different sim cards, whereas same sim cards when installed in other mobile like nokia e52,no call dropped ????????

Any thoughts on this ? I Got 3 bars and boom ! Call failed.. and speaker option also intermitant :((((((

Replace iphone 4s in warranty, reinstall complete window and itune and restore mobile, but problem same ..

in my oponion its IOS 5.1 issue and what you say .... Tireddddd alot