Not wanting to sound like an Apple basher, because i'm not, but if your worried about signal strength then maybe the Iphone isnt the best choice. My company brought everyone the new Iphone when it came out and all I can say is its a nightmare trying to get through a call without it dropping off. I have the Desire HD as do a few others on our company (they gave us a choice) and the Iphone is easily the worst of the two. Even when calling from landlines there are problems. Our company has used both O2 and Vodaphone over the last couple of years with the same results.
Might just be bad luck but i don't think so. The Iphone is a great handset but from my experiance not the best for calls!
Oh and I tend to work all over the place but mostly in London, and Vodaphone very rarely lets me down on signal strengh.