Sorry, what? More than 1 in 4 calls fail and that's normal? In a city?
I know London and New York are similar and different in many ways, but I can't believe it's so much harder to provide coverage in NY.
Here in the UK I spend hours on my phone and travel across London on calls almost every day, with two phones on Vodafone and Orange. On Orange, I can't remember the last time a call dropped, on Vodafone there is one office I visit where 2G coverage is a bit poor, but 3G fine, which leads me to the following non technical guess about the AT&T/iPhone issue....
I have both an unlocked iPhone and an N95. In that office with poor Vodafone 2G signal the N95 is fine as it sits in 3G mode. The iPhone however seems to get confused and regularly drops into 2G mode with poor signal. On a Nokia of course you can also turn off 2G/GSM... but this option seems to be missing from the iPhone.
Could it be the iPhone has a preference for ye olde 2G/GSM and is failing to pick up the shiny new WCDMA 3G (which it seems some new masts only provide), causing the dropped calls?