[vilord]

I've got a verizon phone that did that, it needed to be reprovisioned (quick call to customer service).

I've read elsewhere that the same issue occurs with Sprint, when you run the flash, even if you skip the customizations it sometimes wipes out the auth code and that part would need to be reloaded. Since you didn't write down the one that was in there, Sprint needs to generate a new one, push it to your phone (OTA) and update their servers, which in turn allows calls from verizon.

Why it works on Sprint towers but not VZW towers, I'm not sure, except perhaps Sprint doesn't do the extra auth checking... I can only guess that its something VZW put in place to block ESN copiers.