I have adequate signal on my iPhone 4s, however at varying times throughout the day my phone fails to make calls, receive calls, send SMS or receive SMS. I've tried everything from retsore to even a replacement sim but still no change? Help???????????????

I've spoke to my Network carrier and they have replaced the SIM. This didn't help and this morning i received a replacement new handset, again i have had the same problem. Here's 2 screenshots below of what i mean.

As you cn see my signal strength is adequate but for some strange reason the message didn't go through and wouldn't go though for another hour. I couldn't make a call but asked my fiancee to call me and she could not get through. She also sent an SMS and that took nearly three hours to come through. This problem does not solely occur when i am at home, but has also occured approximately 8-10 miles from my address. This confuses me and i have no idea what needs to be done to stop this from happening. I have restored the phone several times, i have even restored the phone and started up with none of my old contacts, mesages or apps on the phone and that still didn't work. I am getting to the point where i might do the inevitable and get a blackberry handset instead. Any ideas anybody??