The iPhone 4 has been recently criticized for calls dropping out all the time, this is mainly due to the cellular networks not being able to handle the data consumption that these little phones can use up. Everyone is using them for more than a phone but like mini computers. The new screen takes more data because of the higher resolution. In more densely populated areas it is not uncommon to have to wait for the call to connect for up to thirty seconds or even more and thats if you can even get a single. Some say that the antenna used is not good enough, however Apple used the top of the line antennas and it is more likely to be a network problem.
There are some ways to increase the drop out problem, you can position the phone in a different way or go outdoors to use the phone. The easiest way is to change the way you hold the phone so that you don't have a tight grip on the phone, specifically the lower left hand corner. You can also use the free rubber bumper that provided in all the new iPhone 4's.
If you are not getting any service contact your service provider and complain but before you do that ensure that the airplane mode is off as this will cause you to get no service, if this does not work try switching on and off the sleep mode a few times and see if this rectifies the problem.
If none of these tips have fixed the problem you may be outside of the network areas, the no service problem on the new iPhones is quite common and if all else fails contact your service provider and discuss the options available to you. It is likely the problems will persist until the cellular network companies upgrade their systems to handle greater data consumption.