iPhone 4 signal problems

I have an iPhone 4 and everytime I come off the subway, my phone is unable to detect a signal, it is stuck on No Service until I restart my phone.

Here are the things I have tried:

- changed my phone 2 times

- restored phone to factory setting

- used the phone without running any apps in it other than the built in apps

- changed my sim card

- reset network settings

-turn phone on airplane mode before entering subway

This is an extremely frustrating problem, I have called my carrier and they insist there is nothing wrong on their side, and I have called Apple and they insist it is my carrier's problem. Restarting the phone is a temproary solution, but it is unacceptable since I go on and off the subway multiple times a day.

Is anyone able to help me?