I have an old W385 flip phone which was my last basic phone before switching to a smartphone. I will have to agree with that phone had a great antenna.. I don't have a recommendation for you on which smartphone would work best for you. I can say that I seemed to have better voice calls with my HTC Thunderbolt then I have had with my Samsung Galaxy Nexus.
Maybe it would be a good idea to note down which phones you have seen in your job where they don't get signal. That way you can rule those out first and start looking at the phone models that remain.