There is no API to determine the current signal strength of the phone radio on a Windows Mobile Phone Device. But as it changes the status is updated in the registry at the following location.[HKEY_LOCAL_MACHINESystemStatePhone]
“Signal Strength” value represents the signal value (0-100). The operating system uses this value to show the bars on the taskbar representing the current signal strength.
You might want to know in your application when the signal strength changes. Your application can poll the registry but polling is a resource intensive operation and should be avoided. Instead you can use RegistryNotifyCallback function provided by State and Notifications Broker mechanism. This powerful feature is available since WM 5.0 and gives applications an easy way to get notified when status of registry value changes.
Visit the following link for more information and sample code.
Native programmers should check out snapi.h file in the WM SDK. It gives more information about the bitmasks of various registry values.