Here's something to try. On your iPhone preferences, navigate to "General," and then scroll down to the bottom where it says "Reset." Tap that, and then tap "Reset Network Settings." You'll get a warning, and the phone will reset to factory defaults. You'll likely lose some Wi-Fi passwords, but you may also find that your cellular reception gets a lot better.
I have two friends who tried this, and they went from one or two bars to four or five at the same locations. This is the same trick they are doing at the Genius Bar, and my research shows that it can help in many (but not all) cases. We've talked about this fix before, but we have a lot of new readers, and some old ones, who may have missed it. There are also some discussions about this on the Apple support board. If you have seen a drop-off in reception, it's certainly worth a try. If it doesn't work, we're sorry. If it does, well, the price is right for the fix. Let us know in the comments if it helped, or sadly, if it didn't.
Apple iPhone 6
Apple iPhone 4