Multiple iPhones, same problem - signal dropping

I've been an iPhone customer ever since the iphone4, the iPhone that allegedly had signal issues which ironically is the only iPhone i haven't had a problem with.

Ever since i upgraded to the iPhone 4s i've had problems with my signal. I can stand in the same place for 5mins and my signal can vary from full bars and 4g to no service at all. My phone constantly cuts out during calls, drops 3g and 4g connectivity and if a call doesn't cut out it just sounds like i'm a talking to a malfunctioning robot on the other end of the phone.

I went to see Apple and swapped the phone for a new iPhone 4s but same problem. I changed the sim card with my network provider - same problem. I changed networks - same problem. I upgraded again to the iPhone 5s - same problem AND i'm now currently on my third different network and once again, same problem.

I've called Apple support to no avail. They just suggested resetting the network settings, restarting the phone, making sure i have the latest update installed and a hard reset. None of this has worked. I have also set up the iPhone as a new phone several times, unsynced the data being carried over via my google email account and now finally signed out of iCloud but again, same issue.

The only thing i can think of that remains the same across all iPhones i've had is my Apple i.d. Has any one ever had an issue with this? Is it possible it's corrupt? Apple themselves wouldn't entertain the idea.

Does anyone else have this problem? I cannot find an answer anywhere and therefore no solution either.