It all depends on the roaming agreements in place.  ATTWS may have roaming
agreements in places that Cingular doesn't, and vice versa.

No digital carrier has roaming agreements with *everyone*.  Carriers usually
provide coverage maps to show where you can roam.

Which network the phone will select depends on how the phone is programmed (on
the SIM) by the carrier.  There are four different modes of GAIT operation:

     * GSM Native Mode: This is where the mobile is homed to a GSM
  network and is operating on a GSM network. In this mode, the mobile
  is able to perform standard GSM type functions including SMS and Data
  services (circuit switched at 9600 or 14400 bps and packet switched
  service if it is offered).

     * ANSI-136 Native Mode: This is where the mobile is homed to an
  ANSI-136 network and is operating on an ANSI-136 network. In this
  mode, the mobile is able to perform standard ANSI-136 functions
  including text messaging using GHOST SMS and Data services (circuit
  switched at 9600 bps).

     * GSM Foreign Mode: This is where the mobile is homed to an
  ANSI-136 network but is operating on a GSM network. At this time, the
  GSM portion of the phone is operational and the network's
  Interworking and Interoperability Function (IIF) handles the call
  delivery and SMS routing from the ANSI-136 network to the serving GSM
  network.

     * ANSI-136 Foreign Mode: This is where a GSM native subscriber is
  accessing an ANSI-136 network. Again, routing of calls and SMS is
  handled by the network's Interworking and Interoperability Function.

In short, a GAIT phone will be "homed" to either GSM or ANSI-136 (TDMA).
Network selection will be based on standard mechanisms in both
technologies.

Suppose that the GAIT phone scans for service and determines that there
are two networks available for selection, an ANSI-136 Neutral system and
a GSM Neutral system. If the PPI (Protocol Priority Indicator) is set to
ANSI-136 Preferred, then the mobile selects the ANSI-136 Neutral system.
Conversely, if the PPI is set to GSM Preferred, then the mobile selects
the GSM Neutral system.

Suppose that the GAIT phone scans for service and determines that there
are two network available for selection, an ANSI-136 Group A (Home)
system and a GSM Group B (Preferred) system.  The mobile selects the
ANSI-136 system even if the signal is better on the GSM system.

Network selection is a function of preferences configured by the
carrier, not signal strength.  It doesn't matter that the signal might
be better on the other technology.

From my originally TDMA Detroit market, I have been able to roam with my
Nokia 6340i everywhere the GAIT nation map says I should get a
signal-except for a 30 mile stretch of I-77 South of I-81.  This phone
is programmed to seek service in this order: Cingular TDMA, GSM, AMPS;
ATTWS TDMA, GSM; T-Mobile GSM; other TDMA carriers; other GSM carriers;
other AMPS carriers.  I deduced this by watching in field-test mode the
selection process in fields 01-01 (GSM or CCCH) and 21-01 (TDMA or DCCH,
AMPS or ACCH).