In order to monitor the amount of data you send and receive over your mobile network operator from your Android device, Stats is a free application available from the Android Market which displays call and data usage, both cellular and WiFi, as well as SMS usage:

Alternately Onavo is another useful application which can provide real-time usage information by application:

In order to check the amount of data you have sent and received over your mobile network operator from your BlackBerry Smartphone, open the Options menu.
Select Device
Select Device and Status Information

You will see your status screen for signal strength, battery charge, memory, etc.
Although there is no text field to type into, just type BUYR and press enter.
Scroll down to the bottom of the screen.
Your voice and data counters will be displayed:

In order to check the amount of data sent and received over your mobile operator from the iPhone, select Settings.
Tap on General:

Should you be in an area of only intermittent or poor 3G coverage, you may wish to set your phone to 2G only mode to experience a better service until you are next in a better coverage area.

To do this, from the home screen scroll up so that the status bar is selected:

Should you be in an area of only intermittent or poor 3G coverage, you may wish to set your phone to 2G only mode to experience a better service until you are next in a better coverage area.

To do this, launch the Phone application from the Start Menu. Press the Menu button and select Options:

Should you be in an area of only intermittent or poor 3G coverage, you may wish to set your phone to 2G only mode to experience a better service until you are next in a better coverage area.

To do this, launch the Settings and select Cellular

Set the option for 3G Connection to Off

Revert the change to return to automatic mode.

For information on how to perform a manual network roam on the Windows Phone 7 platform, view this article - http://blog.brightpointuk.co.uk/performing-manual-roam-windows-phone-7

UPDATE - this article does not apply to the iPhone 4S: the option to disable 3G is not present.

Should you be in an area of only intermittent or poor 3G coverage, you may wish to set your phone to 2G only mode to experience a better service until you are next in a better coverage area.

To do this, launch the Settings and select General

Should you be in an area of only intermittent or poor 3G coverage, you may wish to set your phone to 2G only mode to experience a better service until you are next in a better coverage area.

To do this, launch the Settings from the main application menu and select Wireless & Networks:

Should you change the SIM in your HTC Android device, or accidentally edit or remove your GPRS / 3G connection settings, you can restore the settings back to their default values quickly and easily.

Launch the Settings from the main application menu and select Wireless & Networks:

Should you be experiencing poor signal strength or no service, it is occasionally necessary to perform a manual roam on your device to de-register it from your network provider and then re-register it again.

To perform a manual network roam on a BlackBerry device, select the option to Manage Networks from the main menu

Select Mobile Network Options
Change the Network Selection Mode from Automatic to Manual