What is 'Manual Network Selection' or 'Select Network?'

Manual network selection also known as "Select Network" is an network settings option on virtually every wireless phone available.  It gives you the option to have your phone scan for and see all compatible wireless networks.  You can then select one of these networks and use it instead of the default or primary available network that your phone would automatically use.  Typically it's not necessary to manually chose a network because your phone does this automatically, choosing the best one.
Cingular has made the business decision to disable manual network selection and force customer's phones in to automatic mode when in the US.  Outside the US manual network selection is supposed to be enabled.  Quite simply the reason they do this is to prevent customers from manually selecting one of their roaming partners unnecessarily costing them money since they do not pass roaming charges on to the customer.

The Manual Network Selection Screen

The disabling of manual network selection is actually done in the SIM with cooperation of the phone.  Basically all phone are required to support this optional feature that when instructed to, they will hide manual network selection from the user as it never existed.  This very simply instruction is in the SIM, and Cingular uses it.  So basically the phone turns on and gets customer and network information from the SIM and along with it gets this instruction to turn off and hide network select.  If you were to put a T-Mobile SIM in r almost any other GSM carrier's SIM in to a phone this option (select network) would be enabled.  But put a Cingular SIM in that phone and this option is disabled.  There are exceptions such as old Cingular SIMs from before they started this practice, AT&T wireless (now Cingular,) and some models of phones that either don't support his or do so incorrectly.

This post by awj223 on Howard Forums describes this in better detail:
http://www.howardforums.com/showthread.php?p=9249845#post9249845
"There are instructions on the SIM known as CSP (Custom SIM Profile) that can tell a CSP-compliant phone to do things such as disable the manual network select menu. The Cingular SIMs, since the 32k ones, have CSP instructions set to disable this menu. They also told the phone to hide true alpha tags and always display "Cingular" using something called EONS (enhanced operator name string) when using a domestic network (country code of network matches that of the United States)."