This seems to be the simplest response i googled regarding that auto A/B query. Copied this from another forum, seemed easier.......
>>Ok this is strange. I was looking on my phone at network settings and
>>saw where you could select from Home only, Automatic A, Automatic B.
>>Now I know when analog was the only thing cell phones used, the wireless
>>cell phone provider had it stored in the phone what setting it was to be
>> on. Now, why do I have that option on my phone? I haven't seen this
>>kind of setting in years. I know Verizon's network is not all digital
>>is that why it's on the phone? Also what happens if I change the
>>setting from Automatic B to Automatic A?
>
> It only has meaning when the phone roams in analog mode AND the
> carrier is NOT in the Preferred Roaming List (PRL).
>
> Most of the time, the PRL tells the phone what carrier to use based
> upon the SIDs (System IDs) that the phone sees in a given location.
When your phone cannot find a system listed in the PRL, you can fall
back to either CDMA digital or analog with the Auto A/
[COLOR=#cc9933! important][COLOR=#cc9933! important]Auto[/COLOR][/COLOR] B setting. In
fact, the phone probably first will try to find a CDMA signal. I've done
it. These are the "old" cellular bands (850MHz), and the early CDMA
phones used these bands before PRL's came out, and thus the phones had
to be able to acquire a system without direction from a PRL. Current
phones will still do that when the PRL lookup fails. What you *can't* do
is fall back to a system on the PCS(1900MHz) band, since the "pilot"
signals there are scattered all over the place, and the PRL tells the
phone where to look for them.