Fascinating discussion! I've been using Line2 for a business phone line on an iPhone 4s for some time now, and I've always noticed that it seems like callers can't hear the first second or two of my greeting. I chalked it up to Line2 VOIP and started pausing before answering. It became so frustrating that I eventually decided to port my business number to a new line of service on our AT&T plan, and instead of purchasing an iPhone 5, I chose the Lumia 900 (cheap, reputation for strong signal reception, and wanted to check out WP7.5 anyway). I'm not noticing this issue at all on the Lumia 900--even for the calls that are still forwarding from Line2 (still have an 800 number with them.) Calls are crisp and clear--noticeably better than with the 4s.
I would prefer to stay on iOS and buy the iPhone 5, but I think I'm getting much better phone service on the Lumia than I would on the iPhone.
(And BTW I remain a big Apple fan--4 iMacs, 3 Airs, 1 17" Macbook Pro, 5 iPads and three iPhones in the family. I'm not trying to stir up a debate--just relating my experience.)